Hi Danny, i'm killing two birds with one stone replying to this post.
I saw you're other post, & i'm in Wistaston, Crewe, so would be happy to meet up for a rideout.
Sounds like you have a big problem with the diff & 230cc is the correct amount of oil, although some have said it has been reduced to 180cc, not that the change would have caused your problem anyway.
Ron, 'drumwreaker' had a similar problem recently, have a look through his posts from earlier this year.
His local BM dealer diagnosed a new diff bearing, but when he took it to a local independant BM specialist it was just a trunion bearing worn which was much cheaper to replace.
I went to Southern France with him a month ago & he had no further problems.
So if the dealer agrees to fix this FOC then no problem, but if he want to charge you megabucks for a new diff, it might be worth getting a second opinion.
Lets hope yours is a simple & cheap fix.