But it’s more likely to be between the compressor and the tank (or the tank itself)?

Could be anything, sounds like you bags aren't leaking but your loosing pressure from the tank -- so simple steps first, check the tank, then the lines Current: 2011 FFRR Vogue SE 4.4 TDV8
Previous: -
2004 FFRR Vogue 4.4 V8 with LPG
1997 P38 4.0 SE with LPG
1997 P38 4.6 Vogue with LPG

It could be the replacement piston seal is either faulty or has been installed incorrectly. I had a similar issue with my compressor and I replaced the original piston seal with a replacement ring off the bay of fleas. Still had the same problems, but when I checked the piston ring fitting in the cylinder bore I could clearly see that the ring wasn't sealing properly, the pressure rose to a couple of Bar when working, then died off (was monitoring with the IID Tool). Got another ring kit from a different supplier and it sorted the problem straight away. You should get up to 10-13 Bar pressure into the air tank/reservoir from the compressor.
My suggestion would be to disassemble the compressor and slide the piston ring into the bore to check it's perfectly sealing. Also check you have the correct orientation of the ring on the piston, as this may cause part of the problem.