Now i'm wondering who i can believe on here - the boiler has been
running for 4+ years in a large 3 bed house with 8 rads connected and
other than the usual moans about having to wait 10 minutes to fill a
bath no one has moaned about the lack of heat output from it.

My initial question has only been promted by the fact that the recent
tennants have left giving me the opportunity to carry out some work -
including (if needed) a remote room stat.

If i undertsand the responses so far i'm looking at having to replace
the boiler - or have i missed the point?