Stuart Noble coughed up some electrons that declared:

Have just acquired an Everest white aluminium door which I thought I
might use to replace my ageing wooden back door. 30mm glazing, so can't
be that old.
It comes with an aly surround, but this doesn't have enough rigidity to
call it a frame. I imagine I'm going to have to construct something out
of wood to take it, but in many ways it would be easier to fix the
surround directly to the masonry top and bottom. Is there any reason why
this isn't a good idea?

I have removed an ali door that is at leats 20 years old that was fixed
directly to brick. The frame is strong but it's not very thick and clearly
relied on being fixed at regular intervals.

If you use frame packers so doing the screws up tight doesn;t warp the
frame, but allows you to get a good hold with the screws, I see no problems
at all. Run a bit of mortar or PU foam around depending on gap size - that
will firm it up nicely, as I found with my new PVC windows (good fit ith 6
frame screwed, unmovable after foaming in).

Cheers

Tim