I do have the Poltergeist playing silly tricks on my NG again today....got to work parked up and tried to lock the car, feeble horn parped when I used my remote...tried a few times and I got the horn blowing. Went round the car and boot was shut and none of the doors were ajar. Tried the remote again 'beep!'
Gave up.
Lunchtime - went to the car, it was locked and started the ignition 'boot lid open'

Had no reason to open the boot this morning, no warnings on my way into work...tried it and it felt and looked shut...

gave the lid a tug and it opened....
Used fob again, no horn...

I was going to take the Golf into work this morning, as Saab was low on fuel. But Openreach are working on our round, thus blocking Madge (the Golf, so called because I had a Dame Edna). Confidence gained by hopefully fixing the water issue without breaking any trim gives me the confidence to try to fix the horn on the NG before the MOT.