Thanks. I'm puzzled by this though, because Article 5 says:
passengers "have the right to compensation by the operating air carrier in accordance with Article 7, unless:
(iii)they are informed of the cancellation less than seven days before the scheduled time of departure and are offered re-routing, allowing them to depart no more than one hour before the scheduled time of departure and to reach their final destination less than two hours after the scheduled time of arrival."
So that's clear I have the right to compensation under Article 5. But Article 7 says:
1. "Where reference is made to this Article, passengers shall receive compensation amounting to [£520 for all flights over 3500km]
2c. "When passengers [on flights over 3500km] are offered re-routing to their final destination on an alternative flight pursuant to Article 8, the arrival time of which does not exceed the scheduled arrival time of the flight originally booked by four hours, the operating air carrier may reduce the compensation provided for in paragraph 1 by 50 %."
So that makes it sound like I'm only owed half of £520.
Edit: Sorry, I realize now that was the EU legislation I was looking at, although I found it on a UK legislation page:
https://www.legislation.gov.uk/eur/2004/261/article/4# Not sure of the extent to which the current UK law is exactly the same or not.