I found this page on the Arriva Trains Wales website while trying to sort out the Club 55 tickets for my parents.
The have a number of partner offers with Club 55 but think this is the best one out of everything listed. They also have simialr offers with the Lion Hotel in Shrewsbury and the Metropole at Llandrindod.
Here's the text from their website:
Cardiff is an ideal location for shopping and first class entertainment, so why not stay over with Sleeperz Hotel for only £55 per night? Must not forget to mention that Sleeperz Hotel is in a prime location, only a minute walk from Cardiff Central Station and 2 minute walk from John Lewis and St David's Shopping Centre.
To book, please call Sleeperz Hotel on 02920 478747 and quote Club55. Show your Club55 ticket on arrival to validate the offer. Visit the hotel website for further information: http://www.sleeperz.com.
Please note: Offer rate will be valid for stays from Monday 04 October 2010 to Monday 31 January 2011 inclusive. The £55 rate is a per room per night rate and includes full cooked or continental breakfast and free Wi-Fi and UK landline telephone calls. The offer will not be valid for stays on any Saturday nights. The offer will not be valid for stays on the following dates, 08 October 2010, 05 November 2010, 12 November 2010, 26 November 2010 and 31 December 2010. This offer is subject to availability. This offer is not available online, please call direct 02920 478747.