filling the gap

Get the builder back to put the correct upvc strip across the top to finish the job off properly. I am sure the strip will cost him all of £2.50.

use foam but use it properly i.e. don't overfill. evostik now do a 2 part foam which sets rock hard in about 1 min and doesn't expand as much as traditional fillers.

Do you have a product name?

yes, but it's in "professional plumber" magazine or summat and that's in the mrs's car. I'll dig it out tomorrow and let you know.

I've used the 2-part Fischer product from Screwfix:

That's not bad. I guess that this uses the same mixing technique as the mortar and epoxy material for their stud fixings....

Yes, the mixing takes place in a long nozzle. This nozzle isn't reusable, so the tubes are pretty much single-use.

Just mask off the surrounding area (window, reveals etc) with newspaper/masking tape and use normal foam, You can cut it back after it has set. Then fit UPVC trim. Simon.

Buy a foam gun - so much better than the cans. I had to use a can again the other day. Never again!

I found that Fischer do a kit of stuff for their fixing system including a bunch of nozzles and various other bits and bobs. I was concerned about the single use thing. In reality it wasn't a problem and it worked out overall less expensive to buy the smaller tubes of gunk and keep the extra nozzles.

BEWARE

I tried this stuff and it set hard in the nozzle within 10 seconds of taking finger off the trigger. Two canisters wasted at £8 each!

I can only assume the temperature (about 12 degrees C) caused the problem.