what would be a very rough estimate for the cost of installing CH?,
Ok now for the other questions

4000 quid. (NOT from BG)

1) To install the central heating and new ring/radial main, I presume it
will be necessary to take up the old floor, how easy is this to take up
and
replace once fitted,

Only some boards will need removal. The vast majority may be left down. Some
will need cutting and they will never be quite as good as they were.

2) The building has old sash windows that I want to renovate, can u get
draught excluders for between the sashes (if that's the correct term),

Yes.

3) What steps would need to be taken to produce a "wet room"
shower/bathroom
up stairs the flooring is wooden floorboards, is this feasible?.

Don't bother. "Wet rooms" in the UK turn out be like fungal growth
laboratories. The floor is always stinking wet and cold. You can't go to the
toilet with shoes on because it gets mud everywhere. You can't go without
shoes, because someone else has gone with shoes on, or you don't like
squelching through an inch of stagnant water in your socks. You can just get
away with wet rooms in sub-tropical areas, such as Greece etc, although even
there you get issues.

Christian.