i changed out the relay and it didn't trip ! this is something that i don't uderstand , i have a bank of 8 second hand relays from a m/c that was decommissioned about 6-7 years ago so maybe it's because the one i used had some mileage up on it, i don't know. but anyway it great to resolve that issue. i had ordered a D rated MCB before i got around to changing it out so at least i will have that as a backup.
on reflection i dont think my latest dwg of the alarm circuit will cut the mustard. my understanding of open collector terminals was not correct, i'm thinking. i have my alarms in the circuit and expected them to work as normally closed contacts, so i will go with the 2nd relay as suggested by your good self Jazz .