Best way to value an engine is to compare it with what's on the market just now. 2 strokes are still sought after as a good no-nonsense engine, excellent power to weight, relatively frugal at this hp, and none of the storage issues compared to 4 strokes when being transported.
Bridger Marine has a couple (difficult to tell if standard or long shaft), priced at £2k.
John Bridger Marine
Bill Higham (expensive at the best of times)
Buy Used Outboards mariner 15hp short shaft, manual start, tiller control 15.0HP Outboard at Bill Higham Marine
Only thing to bear in mind with dealer prices, is that they usually offer a minimum of 3 month warranty.
You can still buy 2-strokes new for around £2,300 (for commercial use only), so at least that gives you a ball-park figure. Yours has got to be worth £1,000 - £1,200 easily.