There should be a fused spur by the panel, if not there is also a mains fuse behind the control panel pcb, where the mains comes in.
Can the cats get within six foot of your existing detectors?
The Gardtec panel is old, and they have never been the best panel. But I am not so sure that other panels are much easier to install, programming wise the 595 is very simple in comparison to many. Newer panels have more features yes, but the more the features the harder it is to program.
As for your no-show, there are several alarm companies out there, I would recommend you look for an approved co, I am not saying that one-man-bands are no good, there are many on here that are proper chaps. But what I am saying is if you use a company that has gone to the trouble of getting accredited they are more likely to show up when they say they will, do what they say they will do, and charge what they say they will charge
Pop your postcode in on here https://www.nsi.org.uk/ or ssaib.co.uk