I am looking at how to design a heating system and there are a few
things that I need help on. If I have the heat loss from a room(W) the
desired temperature of the room(c) how would I get from those figures
back to a flow/return temperature in the radiator?

I know that the rads are designed on a 50 degree temp difference
between the ambient air and the mean water temp in the rad, and that
mean water temp is made up from the flow/return temps but if I was
looking at the design of the initial heat source would I just design
for an approximate heat output and then size the rad around this output
or are there some definite calcs that can be done.

Any help appreciated

Cheers