To me thats not so pretty a rifle to a walnut 55. Its rarity should hold some value. I'll stick with £320 valuation though thats could be a tad high.
In truth its whatever the market is willing to pay and for such rifles the market is small. HW does have a following and to plug the gap in a collection these don't come up often. For target shooters the Original 66 is the better classic. There are just sexier rifles to be had once over the £300 mark.

Very good luck with any sale.