I've decided to ditch the box. A few of my actuarial colleagues (some of whom were involved in the early stages of setting up telematics-based coverage for well known insurers) have all agreed the risk of cancellation is such that I am better off biting the bullet and going to a non black box policy, even if it does mean losing my NCB accumulated for this year. They were all quite surprised this clause wasn't in the contract from the outset (as for whether it was added in May 2018, I am not sure - it's definitely the first time they've mentioned it in a warning email though).
I've managed to find a fully comprehensive policy for £1,300 per annum with a total excess of £1,000. This policy was actually quite difficult to find as most insurers seem to only quote with an excess of £3,000 for drivers of my profile (if you want your premium to be < £2,000 that is). I am hoping they won't turn around and tell me the quote was given in error!