Quote from the link below.
http://www.firstgreatwestern.info/coffeeshop/index.php?topic=3035.msg23283#msg23283FGW rolling stock
All twelve First Great Western single carriage class 153 units have now been refurbished by Wabtec Rail at Doncaster. First Great Western now has eight three-carriage refurbished class 158 units in service – the company is planning to have eleven three-carriage refurbished class 158 units, primarily for use on services between Cardiff Central and Portsmouth Harbour.
Hope this helps.