Roger Mills wrote:
Not exactly - it's not quite linear. The factor given by Stelrad is 0.798 -
whereas 50/60 gives 0.833

I'm surprised it's anything like linear. I would have expected it to be
related to the difference between the rad temperature and the room
temperature - which would mean the conversion is more like
(50-20)/(60-20) or 0.75.

aHA! Found the table! But it's in a PDF.

http://www.stelrad.com/uk/docs/lst_2.pdf

last page (they are *very* big pages!) it's the table near the top at
the left of the rightmost quarter of the page (IYSWIM) and note that the
temperatures are differences to room temperature.

Now I've just found a link suggesting that the return temp. for a
condensing boiler needs to be below 55.

http://www.condensingboiler.org.uk/

so if you have flow at 65, and return at 55, average rad. temp is 60 and
your delta-T is only FORTY...

Andy