Shouldn't be a problem. Only got rads on 2 floors, with the cylider on the
lower floor - I assume that will be OK?

Yes, but you must specify a model that can be topped up from a roof tank and
be vented in the conventional way. The specific DPS Pandora I have wouldn't
work, as it requires to be the highest point in the system. This wasn't a
problem for me, as I'm not running radiators through it, so it is the only
point in the system...

Christian.