One of my articles in AGW about ten years ago took a retro look back at one of these, when one cropped up at an auction (so I test fired it and put it through its paces for the mag etc). The one I featured in AGW was in near mint condition and worked superbly, yet it remained unsold come auction day, as didn't reach its £1,000 reserve, despite being well advertised, and in an Air Gun auction ... So given that top end values haven't increase much since 2008 (+/-), and the fact that the example you are interested in is needing some 'specialist' attention, imho, don't give more than £750 for it, tops. Hope this helps?