I'm looking to move the hot water cylinder from its current location
directly above the boiler (in the bathroom) into the loft. This will
involve a much longer pipe run with a number of kinks, so I'm guessing
I need to convert the system to be fully pumped?

Yes. You need to decide whether you are going to pump the existing gravity
circuit, or convert the system to an S-Plan or Y-Plan zone valve type system
and use the old gravity circuit just for feeding and venting. You'll find
loads about this within this newsgroup using Google.

Christian.