The message
from Dave contains these words:

I can size the rads for a 70 flow but can't see how to achieve a 50
return. If the house is heating from cold (and all TRVs are open) the
return will presumably be much cooler than when the house is up to
temperature and all/most TRVs shut; is it really possible to design for
a specific return temperature?

You are looking at this from the wrong perspective. The 70 flow, 50
return is an element you use in calculating the required radiator sizes.
In operation you won't initially get the 70 flow as the boiler has to
heat the return from cold but you should get (ignoring any losses in the
pipework) a 20 degree rise across the boiler.

If the rise is too low decrease the pump speed, too large increase the
pump speed. Unless you have a continuously variable pump (do such
animals exist?) you are not going to get this exact and as the TRVs
start closing off the circuit resistance will increase mimicking the
effect of decreasing the pump speed so I think you should aim low* if
you have a condensing boiler and want to benefit by recovering any of
the latent heat out of the steam in the flue.

Not having a condensing boiler myself I haven't had to deal with the
practicalities of this but ISTR from long ago that to get any condensing
effect the return must be below 53C and that it is not an on/off effect
but develops over a temperature range.

*This may not necessarily be correct but keeping the return temperature
low is critical in getting the best out of a condensing boiler. No doubt
someone more knowledgeable will be along in a minute to pull my
contribution apart but don't take any notice of dIMM unless he is backed
up by a good majority.

--
Roger