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.

The vent pipe should be on the boiler flow. Some boilers might not require
this, but speak to their technical help line first. The boiler will almost
certainly require an overheat shutdown (i.e. be suitable for
sealed+pressurised operation), but this might not be enough.

Christian.