What do i put in the gap where our wooden framed windows meet the brickwork?
IS it cement or putty?
One gap in particular (front door) has around 15mm gap around where existing stuff has fallen out, i want to get these gaps filled asap.
Sounds like you have replacement windows/doors. Even a 15mm gap will not fill successfully with mortar. Suggest you fit a filler piece in the same material as the frame for gaps this large. For smaller gaps use silicone frame seal.
Expanding foam is good for larger gaps and will also fix the frame securely to the wall. When dry, on the inside rake out a little and fill the hole with filler so you can decorate it. On the inside use mortar and silicone.
Sounds good to me both filling with expanding foam and using a fillet is a good idea, the only thing I would question is the use of silicone to seal, ideal if the frame is UPVC but a painted frame needs acrylic caulk, as I am sure you have all had trouble papering and painting over silicone sealant around the frames on the inside of windows and doors.
My use of avid use of silicone comes from bathroom and kitchen fitting. I must admit to having my wrist slapped in the past for using it in preference to caulking on surfaces that are to be painted which of course you shouldn't.