Is your old boiler not capable of sealed operation? What make and model is
it?

Potterton Netahet 10/16 (approx 15 yrs old but still in excellent
nick)
The manual states that I need an optional "over heat thermostat kit"
which I very much doubt I will be able to get hold of.

Do you plan to have a shower/bath up there?

yes - but I had planned on fitting a small pump / flow sensor if the
water pressure is low.

Ben