Mine seems to be in this category too. As with the OP I bought this back in July so I do not know the age of the battery. Starts ok, but you can sense the crank is starting to slow a little. It of course does not help with not being able to use it regularly.
I have just taken a punt on one of these on amazon
BUDDYGO Car Battery Charger, 12V/24V 8Amp Intelligent Automatic Battery Charger/Maintainer Delivers 3 Stage Charging, with LCD Screen And have 6 Charging Mode, Suitable for More Types of Batteries
£30 and I can always return it if it is not suitable. Says it works for AGM batteries, and I can confirm the current fitted battery is an AGM one too.
I'll let you know how I get on with it.