I have a 12 month B visa - no work permit - UK passport - I must leave by 17 Feb but need anyway to go on business trip to China before that. Am I required to do anything other than simply get stamped out at the Airport to ensure the visa is valid for re-entry?

Depends. If your visa is a multple entry than you can just leave, as long as you return before the date the visa finishes (not the stamp but the date on the actual visa). If it is a year extension than you must get a re-entry permit before leaving.

As long as the VISA is valid, you can travel to Thailand and stay for up to 90 days before you have to leave. You can than simply exit and re-enter the same day if need be to get a new 90 days upon your return.

You would only need a re-entry permit when your VISA is no longer valid and you want to leave with keeping your current permission to stay in Thailand alive.

So the question is if your VISA itself is still valid when you return or not. If it is, no need to do anything. On your arrival card simply fill in your visa number and check that you are given the right stamp before leaving immigration.

Sorry I should have said its a multiple entry visa

Nothing required - any entry before the expiration date gets a new 90 day stay.

Sorry I should have said its a multiple entry visa

Thanks for the info

Cheers

T