Focus Heating goes cold

[rmhodv]

We have an TDDi Focus on a 52 plate, and have the following problem, and was wandering if anyone can help.
You can be driving along with the heating blowing out then for no reason at all the hot air is replaced by cold air.
Then after some time the hot air returns
Has anyone any thought.
Cheers

[DennisTheMenace]

Air lock in the cooling system ? or if you have the climate like i did in my ghia focus its normal acording to the monkey with wds "computer says no"