Vertitas 8C alarm - internal battery keeps dying?

Hi I have a Veritas 8C alarm in my house which was installed prior to moving into the property 4 years ago.

The alarm has worked perfectly until the last month or so. I noticed the lights started to flash differently and the sound of the alarm changed (almost like it was dying). Finally the alarm came to a crashing halt and appeared lifeless. After doing some research I came to the conclusion the internal battery had died, so ordered a new one, then installed it replacing the old one.

The alarm sprung into life, however the alarm would still not set properly. I left the battery in, however what seems to have happened is that it just slowly drained and now I believe the new one is dead.

Not really sure where this leaves me! I suspect that the mains somehow is not keeping the internal battery charged? I'm not sure how I would check this? From what I can see nothing has changed with the mains and I have not knowingly had a power cut. Also there is the issue of when the battery was charged, the alarm was not setting anyway. When I tried to set it, I would put in my code, leave the house but it would just carry on beeping.

Any ideas or guidance would be much appreciated? I suspect that what I can do is probably finished and I need an electrician, but perhaps someone has an idea on how to resolve?

Thanks in advance