Does anyone know how much I can change before I need to start
implementing
things like the door closers on other rooms and the other things that are
required for new loft conversions? I'm going to call the local planning
dept but just wondered what other people's experiences have been?

Basically, as long as the work at no stage adversely alters the
structure or the means of escape in case of fire, then a Building
Regulations application won't be needed.

I think I would like to improve the current fire escape options by
installing
a road accessible velux or similar. There is already a side window but
you need to get through a gate to get directly below it.

However, when you come to sell the house you may well be asked by your
buyer's solicitors for proof that a Building Regulations application
was submitted for the original work. If the work was carried out
after 1985 then you can apply for a Regularisation certificate. You
haven't said if the floor is adequate (not just boards laid over the
original 4" ceiling joists) or if there's a fixed stair leading to it
from the first floor landing, and the stairs enclosed from first to
ground floors. Self closers are the least of it.

There is a fixed staircase leading up to the room and it was certainly made
habitable before 1985.

The floor has proper fixed floorboards - the joists are 6"x2" - and appears
to be very stable. There's no obvious bounce and the lath and plaster
ceiling
of the room below has only a few minor cracks which I take to mean that
there's
not been a lot of movement.

Regularising a non compliant loft may involve a fair bit of work which
you may not be prepared to do, and would probably be more expensive
than starting from scratch. If you go with your proposed option you
should perhaps consider whether it's worth spending that money lining
and insulating that loft if it's all got to be ripped out before you
can sell your house.

Indeed.

Thanks

Mark.