Here is a link to the London to Cheltenham Spa , Bristol & South Wales and Somerset December 2006 timetable.
http://www.firstgreatwestern.co.uk/Documents/Custom/Timetables/December%202006/Book%20A%20REVISED%20(London,%20Cheltenham,%20Bristol,%20South%20Wales,%20Somerset).pdfThe above timetable service pattern (excluding the Weston - Super - Mare / West Country / West Wales extensions) requires 23 "Inter - City" trains & 2 non "Inter - City" Stroud Valley Line units. As you can see , there are long turnarounds at Cardiff Central , Cheltenham Spa & Swansea.
A more efficient service pattern would be as follows :
London Paddington xx00 - Reading xx26 - Didcot Parkway xx41 - Swindon xx00 - Chippenham xx15 - Bath Spa xx31 - Bristol Temple Meads xx46.
London Paddington xx15 - Reading xx41 - Swindon xx10 - Bristol Parkway arrive xx37 depart xx39 - Newport xx02 - Cardiff Central arrive xx18 depart xx19 - Bridgend xx39 - Port Talbot Parkway xx52 - Neath xx59 - Swansea xx13.
London Paddington xx30 - Reading xx56 - Didcot Parkway xx11 - Swindon xx30 - Chippenham xx45 - Bath Spa xx01 - Bristol Temple Meads xx16.
London Paddington xx45 - Reading xx11 - Swindon xx40 - Bristol Parkway arrive xx07 depart xx09 - Newport xx32 - Cardiff Central xx48.
London Paddington xx48 - Reading xx16 - Swindon arrive xx47 depart xx52 - Kemble xx07 - Stroud xx23 - Stonehouse xx29 - Gloucester arrive xx45 depart xx51 - Cheltenham Spa xx01.
Cardiff Central xx54 - Newport xx08 - Bristol Parkway arrive xx29 depart xx31 - Swindon arrive xx57 depart xx59 - Reading xx30 - London Paddington xx03.
Bristol Temple Meads xx30 - Bath Spa arrive xx41 depart xx42 - Chippenham xx55 - Swindon arrive xx10 depart xx11 - Didcot Parkway xx28 - Reading xx44 - London Paddington xx15.
Cheltenham Spa - xx15 - Gloucester arrive xx25 depart xx35 - Stonehouse xx48 - Stroud xx54 - Kemble xx08 - Swindon arrive xx22 depart xx23 - Reading xx56 - London Paddington xx26.
Swansea xx30 - Neath xx40 - Port Talbot Parkway xx49 - Bridgend xx00 - Cardiff Central arrive xx22 depart xx24 - Newport xx38 - Bristol Parkway arrive xx59 depart xx01 - Swindon arrive xx27 depart xx29 - Reading xx00 - London Paddington xx33.
Bristol Temple Meads xx00 - Bath Spa arrive xx11 depart xx12 - Chippenham xx25 - Swindon arrive xx40 depart xx41 - Didcot Parkway xx58 - Reading xx14 - London Paddington xx45.
The above timetable would still require 23 "Inter - City" trains , but with a key difference. The London - Cheltenham Spa frequency would be increased from 2 - hourly to hourly , as recommended by Jacobs Consultancy in the Greater Western Franchise Replacement Outline Business Case Report (pages 61 & 62 of the link below.)
http://www.dft.gov.uk/stellent/groups/dft_foi/documents/page/dft_foi_612538.pdfThis would free up the 2 non "Inter - City" Stroud Valley Line units , thus enabling the following services to be operated :
Westbury 0745 - Trowbridge 0751 - Melksham 0800 - Chippenham 0811 - Swindon arrive 0830 depart 0833 - Didcot Parkway arrive 0855 depart 0856 - Oxford arrive 0909 depart 0911 - Islip 0924 - Bicester Town 0937.
Southampton Central 0837 - Romsey 0849 - Dunbridge 0854 - Dean 0900 - Salisbury arrive 0913 depart 0915 - Warminster 0935 - Dilton Marsh 0940 - Westbury arrive 0944 depart 0945 - Trowbridge 0951 - Melksham 1000 - Chippenham 1011 - Swindon arrive 1030 depart 1033 - Didcot Parkway arrive 1055 depart 1056 - Oxford arrive 1109 depart 1111 - Islip 1124 - Bicester Town 1137.
Salisbury 1215 - Warminster 1235 - Dilton Marsh 1240 - Westbury arrive 1244 depart 1245 - Trowbridge 1251 - Melksham 1300 - Chippenham 1311 - Swindon arrive 1330 depart 1333 - Didcot Parkway arrive 1355 depart 1356 - Oxford arrive 1409 depart 1411 - Islip 1424 - Bicester Town 1437.
Salisbury 1415 - Warminster 1435 - Dilton Marsh 1440 - Westbury arrive 1444 depart 1445 - Trowbridge 1451 - Melksham 1500 - Chippenham 1511 - Swindon arrive 1530 depart 1533 - Didcot Parkway arrive 1555 depart 1556 - Oxford arrive 1609 depart 1611 - Islip 1624 - Bicester Town 1637.
Southampton Central 1737 - Romsey 1749 - Dunbridge 1754 - Dean 1800 - Salisbury arrive 1813 depart 1815 - Warminster 1835 - Dilton Marsh 1840 - Westbury arrive 1844 depart 1845 - Trowbridge 1851 - Melksham 1900 - Chippenham 1911 - Swindon arrive 1930 depart 1933 - Didcot Parkway arrive 1955 depart 1956 - Oxford arrive 2009 depart 2011 - Islip 2024 - Bicester Town 2037.
Salisbury 1915 - Warminster 1935 - Dilton Marsh 1940 - Westbury arrive 1944 depart 1945 - Trowbridge 1951 - Melksham 2000 - Chippenham 2011 - Swindon arrive 2030 depart 2034 - Kemble 2048 - Stroud 2103 - Stonehouse 2108 - Gloucester arrive 2123 depart 2127 - Cheltenham Spa 2137.
Westbury 2245 - Trowbridge 2251 - Melksham 2300 - Chippenham 2311 - Swindon arrive 2330 depart 2333 - Didcot Parkway arrive 2355 depart 2356 - Oxford 0009.
Oxford 0545 - Didcot Parkway arrive 0556 depart 0557 - Swindon 0620 - Chippenham 0636 - Melksham 0647 - Trowbridge 0656 - Westbury arrive 0702 depart 0705 - Dilton Marsh 0708 - Warminster 0713 - Salisbury 0737 - Dean 0750 - Dunbridge 0756 - Romsey 0802 - Southampton Central 0815.
Cheltenham Spa 0540 - Gloucester arrive 0553 depart 0600 - Stonehouse 0613 - Stroud 0619 - Kemble 0633 - Swindon arrive 0647 depart 0650 - Chippenham 0706 - Melksham 0717 - Trowbridge 0726 - Westbury 0732.
Bicester Town 0942 - Islip 0954 - Oxford arrive 1008 depart 1010 - Didcot Parkway arrive 1021 depart 1022 - Swindon 1045 - Chippenham 1101 - Melksham 1112 - Trowbridge 1121 - Westbury 1127 - Dilton Marsh 1130 - Warminster 1135 - Salisbury 1156.
Bicester Town 1142 - Islip 1154 - Oxford arrive 1208 depart 1210 - Didcot Parkway arrive 1221 depart 1222 - Swindon arrive 1245 depart 1255 - Chippenham 1311 - Melksham 1322 - Trowbridge 1331 - Westbury 1337 - Dilton Marsh 1340 - Warminster 1345 - Salisbury 1406.
Bicester Town 1442 - Islip 1454 - Oxford arrive 1508 depart 1510 - Didcot Parkway arrive 1521 depart 1522 - Swindon 1545 - Chippenham 1601 - Melksham 1612 - Trowbridge 1621 - Westbury 1627 - Dilton Marsh 1630 - Warminster 1635 - Salisbury 1656 - Dean 1710 - Dunbridge 1716 - Romsey 1721 - Southampton Central 1732.
Bicester Town 1642 - Islip 1654 - Oxford arrive 1708 depart 1710 - Didcot Parkway arrive 1721 depart 1722 - Swindon 1745 - Chippenham 1801 - Melksham 1812 - Trowbridge 1821 - Westbury 1827 - Dilton Marsh 1830 - Warminster 1835 - Salisbury 1856.
Bicester Town 2042 - Islip 2054 - Oxford arrive 2108 depart 2110 - Didcot Parkway arrive 2121 depart 2122 - Swindon 2145 - Chippenham 2201 - Melksham 2212 - Trowbridge 2221 - Westbury 2227.
NOTES
1) I have not , as yet , managed to ascertain how many extra freight trains are planned for the Southampton - Chippenham corridor. However , the above timings leave plenty of room for extra freight / passenger trains to be added in the future.
2) Weston - Super - Mare / West Country / West Wales extensions would still be possible using the extra stock already allocated for those services.
3) The above timings are compatible with the proposed Weymouth - Severn Beach & Portsmouth - Cardiff timetables contained within the link below.
http://www.savethetrain.org.uk/forum/index.php?topic=410.msg1215#msg1215