Thanks to all for your comments, but I had a result last night.
Battery was on charge all day yesterday as I'd run it a bit low trying to start engine over last few days. I reconnected it and was surprised by a slightly jolty turn over, previously it has spun over quickly without any spark. I released the key & cranked over again, this time it was still jolty, almost stopping but seemed like it wanted to go but like it was shorting out or didn't have enough power. I released the key again & this time if fired & ran!
I've turned off & on several times now, it always starts straight away as before the problem.
I've no idea why this has happened, I