Why do you want to know its value? If this is for an insurance valuation the valuer will charge, as their fee, a small percentage of what they value it at. Insurance valuation is what the item would cost to replace bought in a shop.
An auction house will value it free and give you what it should fetch if you wanted to sell it .
Most local auction houses will value items free of charge. Indeed, even the 'big boys' will. (I've taken timepieces into Bonham's prestigious HQ in London and been introduced immediately to the same guy that people queue for hours to see on the Antiques Roadshow. He was happy to give me a free valuation and quite a bit of background information about the items).
However if you need a written valuation (e.g. for insurance purposes) then you'll probably need to go a local independent jeweller. Their charges can vary considerably, with some places only wanting a few pounds but with others looking for at least £30. Phone first!