Looks like the AC ports are in the very front right and very rear roughly dead center. Having looked at both caps I'm not sure which is the low pressure port. I'm used to having a clearly marked "L" and "H"...also these are plastice and I'm used to aluminum so its entirely possible I'm looking at the wrong thing....which is why I'm posting

 

The Rear Port (Near the Firewall) is the LOW Pressure Port. The diagram in the Nigel Thorley Book is WRONG!!

 

The low side port is usually attached to a much thicker hose then the high size. This is usually the give away.

 

This is from 1997 XK8 Specs: