mike wrote:
Hi
The system is fully pumped H/W & Heating.

I need to move the H/W tank and to alter the pipe runs, is it alright
for the cold feed and vent pipes to be connected to the return pipe and
not to the feed pipe as it is now.

Possibly, unless the relative pressures in the system due to the pump
may cause some parts of the system to be at negative pressure (wrt
atmospheric) which is Not Good. I'd consider converting to a sealed
system (see sealed system FAQ, google...)