Any lovely Xmas cake makers ? Advise please !

[Twosugarsplease]

My mil makes Xmas cake every year, sometimes lovely, sometimes not.
Been to visit today and she has made one that has sunk in the middle, otherwise very tasty though. I thought I would ask on here if anyone experienced can give advise on how to prevent this.
She has an electric oven, don't know if that will make a difference.

she's opening the door too often or taking it out too early.

Ooh, yeah could be a possibility, I'll give her a ring. Thanks for your reply.

no problem! good luck. home made cakes are the best. if it looks like its burning on top, pop some foil over!

She makes the size of a square biscuit tin lissielovett, so how long in the oven would you recommend ?

Too much raising agent also causes cakes to sink in middle!I would say about 3 and a half to 4 hrs at about 140 c -sounds like cake is approx.9 inches square(sorry I know I'm not LissieLovett but do have some experience).