I fit a 1.8b poitive head pump under a bath to supply a bath shower mixer.
The bathroom is approx 3m below the storage cistern which is in in the loft , the hot
Water cylinder is horizontal , on the same floor as the bathroom .
I am now finding , every so often there are airlocks to every hot tap.
What can I do to prevent air being drawn into the system.
The cold water storage cistern is not running dry.

Without seeing the plumbing system it would be hard to say what the remedy would be, but you may need to start with a vacuum relief valve at 6 inches or higher than the hot water tank and perhaps even the storage cistern. Your issue may not be air getting into the system, but rather not enough relief behind the water being pumped.

A pressure relief valve, can this be fitted to the expasion pipe in the loft.

If it can or cannot be fitted is something that only you can answer. I am not there to see the plumbing design. You may need to call in a professional to assess the situation and do the repair. Not all plumbing work is a do-it yourself project. Let's be very clear here, also. A vacuum relief valve and a pressure relief valve are very different with different applications. Without proper diagnostics I'm only guessing as to what your problem truly is.