So, the questions are: What would 'the panel' use to space the frame? I thought I might wedge at each screw/fixing position, then fill the gap with expanding foam (which will be hidden by the architrave). I guess my concern here is that wedges might slip, compress or otherwise loosen over time - I was thinking of plastic ones (which I have in abundance), but I've a feeling that timber would work better. Is there a better way, eg to cut a wedge of timber to act as a fillet behind the frame (tricky to get the taper right)? Would this be a good use for adjustable screws? I've never used them before, but screws would be more appropriate than frame fixings here as the corner of the adjacent wall (where they will go) seems to be plaster over timber rather than brick and, I assume, adjustable screws would allow me to pull the frame into plumb, if necessary.
On another note (related only by being in the same job!), I shall soon be fitting a toilet and previous posts here have recommended bedding in silicone rather than screwing down. This seems well worth trying but there are approximately a squillion different types of silicone, so I was wondering if anyone could recommend an appropriate one to go twixt bog and tiles.