Just updating this thread, haven't had a chance to get to my sister-in-laws house for a long time.
Anyway, I've spent a few hours on it today armed with all the tips I've got here.
Things are looking better now.
Testing the global tamper pairs, there is definitely an issue to the door sensor, that cable will need to be replaced, will have to wait a few weeks until I can get the carpet up but shouldn't be a problem since they have a suspended floor, so for the moment the global tamper is bypassed with a link.
Next was to add a new sensor that the builders ripped off (Z5) and then couldn't find, connected, did a walk test and seems to be fine.
I reconnected the kitchen sensor Z4 that I suspected was a problem and trying a walk test, it was permanently sounding on that zone.
I tested all pairs and all looked fine with the cables so I swapped that PIR out for a spare I had, all looks good now when doing the walk test.
So hopefully once I swap out the cable to the door sensor we should be 100% working.
I can't believe how many different issue's the system had
Appreciate all the advice and help here !!