the way I see it I'm leaning towards 3rd party option.
My reasoning is that say the bike is stolen, then the insurance company might consider it worth say £5000. I have to pay £1300 excess which leaves £3700. But then I've also paid £1200 more than 3rd party so we're down to £2500 difference to not having theft insurance.
But if I make a claim I'm probably in a much worse position trying to find insurance next time...