It seems that the dealers don't change the £3 seal, they only change the £600 Bevel Box. Changing seals is only possible for owners doing it themselves, yours is under warranty at least. The new box will come fitted with a cheap Nitrile seal that is very marginal for the temperature that the BB reaches, a Viton seal (at about 4 times the price) would be a much better bet, something to keep in mind once the warranty expires.
Instructions on how to do this fairly straight forward job are on the MTWC website.
https://www.mtwc.co.uk/news-post/ I know I keep putting this link on here but there is quite a lot of information on there now for the M3W and many of the problems and questions that come up on this forum are covered on it.
