Veritas Alarm wont set

You need to check each pair of blue and whites , one or maybe more will be open !

Ok, I’ll do that, is there any particular cable I should get to replace the broken stuff, I see a lot of solid core but I’m sure I read somewhere not to use that but I could be wrong.

Is this stuff ok?
https://www.tlc-direct.co.uk/Products/CALV8.html

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 !!

I can't remember if you said some of the cores here were doubled up? I had one (or possibly two) damaged cores (also a history of rats...) to a Texecom PIR recently but as the spark had run 8 core, I had two spare cores so could run these in place of the damaged two. This solved the tamper fault and got everything up and running again.

I understand (from what I read at the time) that magnetic contacts only need 4 cores; if you have two spare cores maybe you can use these (after testing for continuity on them)? This would save having to run any new cable...