Hi, can anyone help?

I am told it is quite straight forward and I am a competent DIY'er so
am very keen to give it a go.

My House was built approx 1930 and has 75mm wall cavity. It looks as
though someone has replace the frame and door at some time in the past
as the frame is between the plaster on the inside wall skin.

The outside wall does not continue under the door with any DPC as it is
just concrete which is part of the front step (same height as DPC). I
dont know how far under the door the concrete goes but I am assuming
there is a gap between the concrete step and the internal wall.

The internal wall door opening width from plaster to plaster is 910mm
and from DPC to plaster at top is 2170mm. I could take the plaster off
the intenal wall which would make the opening approx 930mm x 2180mm.

The outside opening is 850mm from brick to brick and from step to door
lintel is 2170mm.

Some people say bed the cill on motar, then fit the door to cill, some
say screw the cill to the bottom of the door then be the whole lot onto
a couple of beads of low mod silcione.

If the 75mm UPVC frame is fitted to the gap on the inside wall, it will
not bridge the gap and butt up against the outside wall. Should cavity
closers be used? or is expanding foam ok? I assume that the UPVC
finishing trom will cover the gap anyway.

Any help would be good or an illustration would be fantastic. I have
trawled the web but couldn find anything.

Thanks again in advance

Jimmy