not really in tandem as I intend to isolate one from the other :-p

I've just bought a multifuel stove, on which I burn logs, scrap wood and
coal, I've not had the (condensing) boiler on for 2 weeks now and this was
my intention - to save on gas bills.
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 - 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).

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

I don't want to get rid of the combi cos I may need it in spring and autumn
when I don't want to make a fire just to take the chill off a few rooms.

I've run the idea past a few plumber aquaintances and they think it's
do-able

Ideas and thoughts anyone?
And please no lectures about regulations etc