Been having similar probs of late. After a few days on the optimate I wheel it out and it fires right up. turn it off , go in for a cuppa and get gear on only to find it then needs jump starting to get it going again as it won't turn over. I've also had a few strange electrical things like the front side light coming on with the brake light as well.
Well I tried to start it today and it's not having it again so I decided to give the starter a once over. The terminal was properly rusted so tried to disconnect the lead and rounded the nut off

OK take the whole thing off complete with lead and do it on the bench. An hour of swearing, scuffed knuckles, an old torx socket, hammer and plenty of wd40 got the nut off. Took the starter apart and cleaned it up in side a bit the reassembled and refitted.
Key in and press the button and it fired straight up

so I'd say it's worth cleaning the connection on a regular basis even if you haven't got problems.