Dave Plowman wrote:

The new frame comes with a sill - I've specified a aluminium
one as it is lower) Obviously that has to be well supported and level.
What would be the best thing to go between the new and old to support all
over and seal? Is foam strong enough? Or will the wall fixings etc be
taking most of the weight?

The back door of my garage is a PVC frame, with fanlight and door, I
specced a low-level cill, to make wheelie-bins etc easy to get through.
I don't think the alu cill takes much of the weight, couple of screws to
keep it held down, I used a polysulphide sealant to stop rain getting under.