Based on the history of the car, sounds like battery to me.
Worth checking earthing just in case. If you have the steel sheathed heater control, if it is getting hot enough to burn you, that would indicate a poor earth (donâ€™t ask how I know!). MMC fit a flimsy earth from gearbox to chassis. For what it will cost, it is worth adding an extra earth strap in the engine compartment where you can easily check it.
In your shoes, I would fit a new battery anyway. Our 1985 4/4 was never hooked to a charger and the battery lasted around 7 years. Always started easily, until the battery started to fade. The code number on the battery and a tape measure should suffice to get the correct battery. If yours is fitted under the bonnet (4 seater IIRC?) then easy to swap. More modern cars with a constant current drain through ECU and immobiliser definitely need a float charger. With your use pattern, you shouldnâ€™t really need one.