The standard open return fare from Cardiff to London Paddington is £158. You have to use this fare if you want to leave Cardiff before 9am - which of course you must do if you're off to London for work on First Great Western - even if you can book in advance.
So here's the solution (this is up to date as of 20 May 2008 with the new Simpler Rail Fares scheme in place) :
Book a standard advance single for £5 from Cardiff to Swindon for the 7:25 train or earlier (book online any time up to 18:00 the day before). Then a standard advance single for £49 from Swindon to London Paddington, staying on the same train ! This gets into london at 9:29 leaving you a full day for meetings or exhibitions or a day out. If you don't mind changing trains at Swindon (same platform) and waiting 4 minutes you can get the swindon-london single for £41. If you don't mind getting in until 10:40 its only £15.50 in which case get the 8:25 from Cardiff (still better than waiting until after 9).
As long as you avoid the return trip leaving Paddington during rush hour, you can get a single all the way from Paddington to Cardiff for £28.50 at 16:45 or 19:15 or cheaper still £15.50 leaving at 20:15 or later. Use the money for a slap up meal out with the spare time. If you must come back during rush hour you can get an open single back for £79.
In summary
Cheapest option £5 + £15.50 + £15.50 = £36 (total saving £122) outward 8:25 - 10:40 return 20:15 - 22:15
Early option £5 + £41.00 + £28.50 = £74.50 (total saving £83.50) outward 7:25 - 9:29 return 16:45 - 18:45
What ever you do, don't book this on thetrainline.com because these guys add a booking fee. Instead book it on First Great Western's own site www.fgw.co.uk which uses the same underlying site implemented by thetrainline but with no booking fees.
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