[stuart bowes]

a number of options but basically it boils down to, get a decent fault reader, get VIDA, or find someone near you that has one of the above and ask them nicely if they don't mind helping you out

sounds like you have checked the usual stuff already ? but intermittant faults may not show up when someone's testing things so who knows

presumably you did check the centre console 12v socket with the ignition on? if so it's possible it just hasn't been reconnected at the rear after someone did a bit of fiddling

the alarm message may not be as simple a case as you might expect, in the past it was generally considered a straight forward new battery job but recently I've read posts where people have bought a number of 2nd hand units online and found them all junk when cut open. a fair chance your original unit is the same, the PCB all covered with battery juice and burnt out . you can still get the replacement alarm modules though albeit at the cost of near as dammit £200

fuel cap lock is reasonably straight forward, open up right side panel in boot, unbolt and remove lock, open up and cleanout / grease / check operation (and remember it only locks after 10mins unless someones changed the programming at some point)