Thanks for the speedy responses - much appreciated.

So I have a max of 135mm between floors and you are recommending a
70mm min screed which leaves me with the following options:

70mm screed -- 65mm insulation
75mm screed -- 60mm insulation
80mm screed -- 55mm insulation
85mm screed -- 50mm insulation

I guess I can look for some 65mm or 60mm insulation but doubt I will
find it.

Am I ok with 50mm insulation ? (Regs are a bit different here in
Channel Islands to UK so I am not required to put any in - I just
wanted some to take the chill off a bit).

I'll use some offcuts to go round the perimeter but just cut them to
20mm and go full screed depth. Is there a max on this perimeter
insulation? ie would you gain by having it 50mm? Or is this determined
by the plaster and skirting board width/depth? ie a limitation of
about 20mm.

As for the fibromesh - haven't heard of this so will have to
investigate locally.

Chicken mesh or a thicker gauge mesh? Local builders don't sell D49
(2.5mm bar with 50mm squares) but do sell either chicken mesh rolls or
A142 (6mm bar and 200mm squares)

Was in two minds whether to screed with a mate (and get mix delivered
by big mixer or dumped into the back of his truck - he is a stone
mason and has done floors before). Or to pay some plasterers to do
it.

I understand the wood and bay approach but am not convinced I can get
a decent floor with no dips or hills. Did the slab and blocking
previously myself.

Thanks for any help.
Ed