Many thanks for all the help, I managed to view PDF document ok,

I understand the need to have the frame true and square, but should I
fit cavity closers?

The PDF document says butt up to plaster line but the internal wall is
bigger than the external wall so my door will have to be fitted from
inside (it will open inwards). Should I take the plaster off the inside
wall and screw frame direct to the brick without too much of a gap?

After I have squared the frame between the brickwork on the inside
wall, the 70mm frame does not reach the outside wall skin (I know I
said 75mm - I am competent, just illiterate!)

Do I just fill the gap with expanding foam or do I need to line the
inside of the external wall with timber to butt the UPVC frame against.

Or just hide the gap with UPVC finishing strips.

many thanks
Jimmy