I have used my siam city card in the uk before with no probs. Now machines in the uk (nationwide) accept the pin but after pressing the amount the card ejects with a message ' temporarily unable to complete transaction'. Tried AM and PM in uk but with the same result, staff in NW don't know why and say it must be the Thai Bank. Anyone else have this problem of Thai cards not working recently?

Bangkok Bank has had serious server problems for the last day or two and perhaps they handle that small banks overseas communications? I would give them a telephone call during business hours and ask if it continues. Sounds like the UK computer knows it is not connecting rather than anything wrong with your account or card.

Off topic slightly but my ATM card which states "Use only in SCB Atm's" works in Barclays machines in UK !!

I have had the same problem here in Thailand on a regular basis and recently in Europe using my Nationwide card. Used it later and it works okay

Try a different bank ATM, it may be a problem with Nationwide.