effortless is right - how the CAA view the flight is not related to the insurance. If you're taking a club plane to maintenance for the flying school, the insurance isn't going to stop you because you're not paying - they're just going to be interested whether you are licensed to fly it.
I've ferried planes for my club to maintenance, and wasn't charged anything. I don't see that this is any different to borrowing someones plane, and I can't see how the CAA could argue you've been paid - you haven't received any cash. In fact I wouldn't expect to pay anything - you're doing them a favour.
On one occasion I did go with an instructor to pick up a plane, which I wasn't licensed to fly, and he let me take the right-hand seat. However, he said before the trip that the club owner wouldn't be happy with me logging it (I would have to have logged PUT) as if I did that he would expect me to pay, rather than just hitching a freebie. I know how I log it isn't really anything to do with him, but I decided for the sake of future favours to go along with this.