Thanks for your replies!
My utility room contains the 4 pipes as previously described. Was hoping I'd be able to convert without running new pipes but it seems that may not be possible.
I've knocked up another pic. I think this takes into account your recommendations and would mean...
Hi,
Recently moved house and the oil boiler has packed in already.
Current system is a gravity system, so 4 pipes from boiler (feed and return to tank and feed and return to radiators).
I'm planning to get a condensing oil boiler to replace it but need to modify the pipework to make it...