.........I have put the meter on the tamper contacts on the landing pir after joining the wires together at the panel. It read from the pir 2 ohms then when i tested it again at the panel end it read 22 ohms!
Could this be because i have had the front off or Could this be the fault?
I assume you connected them back into the pir then metered them?
If so it does sound like you have found the problem.
The problem being you have an intermitant tamper.
I should also point out that as all the tampers are in a loop (in your case) , at no time was this pirs's tamper ever not working, you seem to have been lucky it did not go off (but that is waht intermitant faults do)
From whtay ou have said it sounds like its the pir tamper switch.
dissconnect the switch for say a week and see what happens.
But you now see how useful a meter is.