However, if your boiler controls the pump in order to provide
pump-overrun, you're more likely to need a by-pass valve on
an S-Plan system.

Yes, I should have mentioned the larger likelihood of needing an automatic
bypass valve with S-Plan. Then, of course, many Y-Plan systems need them
these days, particularly with the increasing use of unvented cylinders which
can't be used as a boiler heat pump overrun dump for safety reasons.

Christian.