It's worth advertising it in the For Sale section here.
Looking on Autotrader, Pistonheads and MMC Used Car search will give you a rough idea of what it's worth, but it can be difficult as these are niche vehicles. You also need to factor something around £3-5k difference between a dealer and a private sale.
SOR through a dealer would perhaps be an easy option as they would store, market and provide security of payment. If you bought it new then the supplying dealer would be my first port of call, followed by whoever did the servicing if different.
By the way, don't bother with "We Buy Any Car" as they don't buy M3Ws!!!
Good luck!
