Being as David Tennant has already recorded the Christmas special for later this year (er, Christmas time), then whatever happens in Saturday's episode, David Tennant is going to continue as the Doctor somehow.
Thanks tony and james, I don't think I have made myself very clear - my fault!
As it seems almost certain that DT remains the Dr, then the regeneration must fail or be halted. I think that the hand in the jar will somehow have a part to play in stopping/reversing the regenaration. I hope that is clearer than mud LOL.
Had sherry now so brain kick-started since this aft.
I think DT's regeneration may have something to do with the Daleks taking the planets " out of time " or whatever the phrase used was.
No way will there be a new Doctor yet.
Ps - I've just been to the Doctor Who exhibition in Cardiff, it's worth the fiver admission to see a lot of original costumes and offers plenty of photo opportunities !