My view is that it's worth what someone is prepared to pay, and that a lot of the value lies in just how desirable the car is. If it stops people in their tracks and is always being photographed then it's going to command a significantly higher price than 'just' another Lotus Seven clone.
I would suggest that, with the M3 motor and mean looks that it has, your Marlin is just about as desirable as a contemporary Marlin gets which should, if it's well advertised, make it much easier for someone to fall in love with it and want it so much that they'll pay for the privilege of owning it.
I don't think you're too far wide of the mark at £20k. To me that looks to be about £5k or so short of its likely true value.
Enjoy the summer and then good luck with the sale