Fitted the other door frame today. As luck would have it, all bar 2 of the
screw holes in the left side of the frame lined up with every faulty brick
(frog voids, cracked brick, mortar joint sans half the mortar etc).

Not having time to mortar up the wall, wait a day for it to harden then
drill the holes, I came up with an interesting bodge involving a leftover
bit of resin mortar (chemical anchor stuff).

As I was using long Fischer plugs, I was able to get at least half the plug
into the brickwork in the worst case and all in but very sloppy in other
cases - so I filled the holes with resin gunk, then inserted the plugs then
squirted a bit more around the plugs that were only half in.

Waited 10 minutes and fitted the frame. Every screw did up rock solid. I
then packed some 3:1 strong mortar in the voids and made sure it was well
packed around the dodgey plugs. It's not falling off now.

I'd not thought of using resin in this way but it seemed very successful.
This was for lateral support - I might be more cautious if there was going
to be a heavy load pulling on the screw though, but for a loose plug it
might still work as long as the resin was sticking to something solid...

Cheers

Tim