Great, thanks mate.

Are the PSI tolerances that vague? Seems a radical difference in poor-very good.

Thanks, Alex

you might find that you have feck all compression but there will still be some as it is a range of efficiency.

less than 100 and you will find that you have properly knackered vales or rings. Slightly higher than that and you will have something you can fix.

The Haynes has a good description of checking them and ID'ing the location of the fault.

Lets hope it's +100 then as something fatal would be a kick in the fanny.

I like to see all 4 readings the same or close, even if a bit down but in spec [185 +/- 28psi at 400rpm.]

All a bit low means general wear, one pretty low means a fault.
Get motor to working temperature, remove plugs, kill switch to OFF, and open throttle wide.
Spin over on starter and check each reading in turn.

If low pour a few drops of oil into the cylinder, if better, suspect piston/bore/rings, if no different then valves either seating or clearances.