On Monday, 18 March 2019 14:15:49 UTC, JimK wrote:

I would be doing something similar in your situation. Screws into
the lintel sounds good to me plus foam if you like but brace it
well whilst it cures or it will bend the insulation &/ or
plasterboard whether it's "low" expansion or not...

DAMHIKT

:-)

Yes, will probably take that approach. If nothing else I'll learn something along the way.