Sounds like 2-3 of the cells are shot.
Previously I used a Numax 75ah leisure battery.
I went through 3 of them each lasting between 6 and 9 months (charged regularly with a good quality intelligent charger).
In my opinion the Numax batteries are inferior quality.
The last battery I bought was a Halfords 75ah, which is noticeably heavier (hinting that it has a higher lead content).
It has been faultless in the year or so I've had it and continues to be so.