1. Once I have taken off the old stuff , I will need to back-fill the
now exposed brick - do I have to plaster ? Alternatives ?

No. Don't plaster to the bottom, or it will wick up any moisture or be
cracked when the floorboards flex.

2. I notice from postings that some people have glued skirting - is
this really going to work with 145mm stuff ?

No. Use screws if possible. Then your expensive and labour intensive work
cutting the boards isn't wasted when you need to remove the skirting to
install flooring or chase out electrical runs etc. Nailing is a right pain,
especially in hard brick or concrete block. Gluing doesn't handle warping or
uneven surfaces well. Both nail and gluing will likely lead to destruction
of the boards if removal is required.

3. Along one wall, I could lay a 4m length - is this wise i.e.
warping, cupping etc ? Do I need to acclimatise the wood as one does
when laying T&G flooring, for example ?

It is best to. However, with enough screws, even a warped board will be held
flat. It is better to use a complete length than attempt to butt join, which
always looks pants.

4. And as to using a Mitre saw to get good joins...

Use a mitre saw for outside corners. Use mitre glue. This is a special
superglue that you get a perfect external corner with. For inside corners,
cut one board straight and scribe the other board to fit over it. You will
have a much better join with smaller, more fillable gaps that give you some
leeway on the exact board length, especially on the straight cut board.

Christian.