Time to invest in a CTEK battery conditioner Simon
Incidentally, during my ownership of the car, I replaced the battery twice over the 13 year period. In Nov 2003 and late 2011/early 2012. The latter was one of the heavy duty ones from Halfords. For the majority of the time the batteries were on trickle charge via the CTEK
It is well known for some cars (Rambo included) that over a period of 10-14 days they discharge slowly if not used - possibly the alarm, immobiliser, radio etc... or something else drawing charge from the battery. That's why I've always found the battery conditioner to be the best solution as it keeps the battery in tip top condition ready to rock and roll
PS I've read that if you leave a modern type battery discharged for several days or longer it may never recover, or only partially recover at best