On Sat, 3 Dec 2011 19:15:12 -0000, Phil L wrote:

Living room is toasty warm but the upstairs is cold, so my intention is
to take a flow and return, attach a pump and rig up a back boiler

There must be a gravity loop from a stove boiler to somewhere it can
dump the heat. This is normally a radiator that will gravity feed
from the stove boiler but controlled by a thermostatic valve that
opens at 90 to 95C. The gravity loop ought to be plumbed in 28mm
copper. The CH circuit will need to be pumped of course but that is
seperate to the circuit removing heat from the stove boiler.

I have access to a heat exchanger from a swimming pool, it's about 2ft
by 1ft, stainless steel and has fittings attached which I can use to
connect the pipes, my intention is to install it adjacent to the back of
the fire (the fire is basically a cast iron box with a glass door and a
hole at the back).

"Adjacent" implies a gap, to get decent heat transfer it needs to be
in good metal to metal contact with the back of the stove. Bear in
mind that if the boiler is doing it's job you may well get tar build
up on the inside back of the stove, this may or may not be a "Bad
Thing".

Im intending to put isolator valves on the flow and return at the boiler
so that it's out of the picture when I'm using the back boiler.

Ideally you have a "neutral pressure point" where the flow/returns
from the two boilers and the CH all meet. By all measn fit isolation
valves for maintenance reasons but having to manually adjust valves
to get safe/correct operation of the system is just asking for them
not to be set correctly at some point.

A commercial "neutral pressure point" device is a Dunsley
Neutraliser, other makes are available. You probably aren't going to
get very much heat from your lashed on boiler so a thermal store
providing the neutral point is not likely to be worth installing.

I gather I'll need an expansion tank in the loft, and the pipe leading
to this ...

Two pipes one feed and one vent. A metal tank is recommended as an
"enthusiastic" firing of a stove boiler can make things *very* hot.

will also have an isolation valve, so that I can remove this from the
system when using the boiler and the fire would be obvioulsy out of
action also.

A valve on the feed is OK but not on the vent.

--
Cheers
Dave.