Was there any particular reason you fed the rads directly from the
primary?

Yes, the radiators are (or will be when I convert the loft) the highest
point in the system. This means that they require a sealed pressurised
system. It is actually possible to have a sealed pressurised system fed from
the heat bank, but this requires 2 additional pumps and an additional plate
heat exchanger. The boiler and cylinder are already in the loft and I didn't
want additional tanks above, even if they could have been accommodated
somehow.

Christian.