Contract question

[BEP]

I have a query....I've been lucky to land a new job and have consulted my contract which states 4 weeks notice, now this is a fairly new contract which was issued when our company got bought out last year. My old contract stated 12 weeks....new contract is fully signed/dated etc etc....now to my mind from a purely legal perspective I would only have to give 4 weeks notice yes?
Our MD has decided that ''it was a mistake'' and I have to give 12?? (i'd given them 8 incidentally)...he's also openly admitted that the company don't have a copy of my newest contract !!

Any thoughts? legally speaking I can't see present employer has a case if i'm honest?