When is a decision not a decision?

I can't quite get my head round this latest Julian Assange business.  He has applied for, and been granted bail, and that decision has been appealed.  So far, so good - but why on earth is he in jail?  He has been granted bail, and unless and until it is overturned on appeal, surely that decision stands?  Assuming he can meet the conditions associated with it, he should be out on bail - so why the hell is he languishing in Wandsworth nick?  Can anybody out there enlighten me?