I've got two MC28 SP's and had one before this and I'd say you got a bargain. I almost bought it myself cos I though it such a good price. The important thing to remember here is that you have a true collector’s bike. The last of a line and I seem to remember a minter at that. The value will never go down not like the MC21's which are ten a penny at the moment.
I sold my first MC28 SP for 1700 quid about 4 years ago. Only one guy was interested and he pretty much named his price. Saw it for sale on eBay last year (now an SE) and it went for £2500.
If you've got one....hold on to it.