Alarm Engineers, need help with Veritas Alarm Bell Box

Hi Everyone
I wonder if you can help. My home has a Veritas 8 alarm system, it's been installed now for over 10 years i believe.
All has been fine for the last few years but recently things have started to go wrong.

At first it was the keypad, only half of the numbers worked. Out came an engineer who replaced the battery in my main alarm unit. All seemed fine for a few weeks when the keypad failed again; back came the engineer who them replaced the keypad.
But now, over the last couple of months, my bell box outside has started to make beeping noises (probably a beep every second) at completely random times of the day or night. Sometimes it can last a few seconds, the longest seemed like 30mins + and at times it sounds like it has run out of steam and the beep elongates and goes down in tone (like someone has pulled the plug on a record player - sorry thats the only analogy i can think of).

My system is unset at the time but as i'm out during the day I don't really know if it is doing it while it is set.
The keypad, control box and internal alarms are all quite, no tamper lights are on the keypad.
I have no way of turning the beeping off; i tried taking the fuse out but that tripped the whole alarm.

Another engineer has been and checked the alarm unit and said the connection to the bell box battery is fine and it is charging as normal; he didn't actually go and look at the bell box and seemed to blame my neighbours alarm (there bell box it about a 1mtr from mine) claiming mine "wouldn't make that noise".

If someone can help me figure out whats going on it would be appreciated, I can then confidently tell an engineer what I want them to do.

Thanks in advance.