During winter months I attempt to start my car up once a month the realise its two however, it just about fires up, The other day I walked the battery to the guy up the road who tests batteries and said mine was find (it just coming up to the end of the three year warranty), he would say that or if dead your buying a new one!
So, I stripped down a spare starter motor ie unsoldering the solenoid coil wires cleaned up the copper contacts within and tested it, appeared to be ok albit the test battery was taken off a car last summer and destined for scrap (££), put it on the car and it just clicked hence put the other one back.
Thinking why I'm always having this problem I invested a new earth strap (£3.99 delivered, 3Awg 9" long), the original goes around the engine mount exhaust side and was the original 1973! Should have changed it years ago, there is a definite increase in starting speed.
Happy motoring