Knocking holes in the wall for extractor fans

[lethargic_man]

I have a running problem in my kitchen with mould on the walls during the winter. I suspect it's because I don't use the cooker hood fan while cooking, but even if I did, it's just got a filter in it: there's nowhere for the steam from cooking to go. It seems to me I'm going to have to get a new cooker hood with a proper extractor fan, and that, it seems to me, is going to require knocking a hole in the wall to let the extracted air out. (Either that or the window, but the secondary glazing would make that problematic.)

Does anybody with experience of these things have any comment to make on this? And of my London readers, can anyone recommend me someone to do the appropriate hole-knocking?

[ETA: Oh, I hadn't realised I'd blogged this already. Though the solicitation of recommendations still stands.]