Boiler is Potterton Netaheat running nat gas. According to the potterton
website the boiler can be be gravity or pumped.

You're laughing then. Run in whatever way the plumbing works best. Ensure
there is a direct unvalved path from boiler output to vent and a direct
unvalved path from water inlet to boiler return. Then place pumps and valves
where appropriate. You can share valves or use separate as you wish. You can
run each of the 28mm or 22mm system as your desire. Pumps don't count as
valves if in the water inlet path. A fixed (non automatic i.e. gate) bypass
valve counts as a path for the water inlet. If the boiler has an additional
manual reset overheat cutout, then the water inlet restrictions can be
ignored. The water inlet could even be combined with the vent under these
circumstances, which eliminates pumping over.

Christian.