1 Will my boiler cope with pumping water to the loft for the
bath, shower & radiators?

No. As it stands, this won't work. The hot water system will be gravity fed
from a cistern. The circulating water will be fed from a small expansion
tank. Both are probably on the loft floor and would provide no head. The
tanks are likely installed in an area you intend to use for the conversion.

To get hot water up there, you should replace your hot water cylinder with a
heat bank or unvented cylinder. Your house is basically too big to seriously
consider a combi. At the same time, ensure that all cold taps are fed from
the mains. This allows you to discard the old large cistern. This all
assumes you have good mains pressure and flow rate.

To get radiators up there, you can do one of two things. (1) raise the
expansion tank on stilts above the radiator level. (2) convert to a sealed
pressurised system. This solution is much better, as it is more reliable and
doesn't take valuable loft space. I don't know if the Kingfisher II is
capable of sealed operation, though. My mother has a Kingfisher running
sealed, but I think it may be a more recent model.

2 Will my boiler cope with the additional strain of 3 rads in
the barn?

Probably not. The Kingfisher is a fixed output boiler. The output would have
been chosen to reflect the likely heating requirement of the house. Unlike a
modern modulating boiler, it won't just work harder because of the increased
need once it starts going flat out. So, for this reason, (and possibly to
enable sealed operation) I'd recommend swapping with a modulating condensing
system boiler.

3 Can I add thermostatic rad valves throughout? If I want to
change them from the yorkshire type, will I have to change the
microbore pipe?

No. You can keep microbore if you like. However, you may have to reroute
slightly if you want it to enter at each end. An alternative is to keep the
old valves and use inline TRVs on a convenient bit of pipework.

5 To convert the 35' room, I'd like to plasterboard it - maybe
with some sort of insulation on the back of the board - is there
anything in the market place like this?

Use Celotex or Kingspan. As you have huge rooms, 50mm is good. If you were a
bit tight for space, then 25mm would make a big difference, too.

The long room will need maybe 3 radiators (currently has 3 non-working
storage heaters).

For such a huge 60' room, seriously consider using fan convectors instead of
radiators. These are like fan heaters than run from the central heating
instead of electrically. The fan assistance will help circulate the air in
the room much more quickly, leading to a faster, more efficient warm up and
reduced set back requirement. A couple of Myson Lo-lines should be fine,
running off their own zone valve and a programmable room thermostat. The
bigger Lolines have 5kW output, a couple of which is probably far more than
the room will need after it gets 50mm of Celotex. However, I'd want to
grossly oversize such a room to gain rapid warmup due to shear thermal
inertia of the air, walls and contents.

http://www.myson.co.uk/pdfs/Fan%20Con%20Tech.pdf

Christian.