>>> . . . and would prefer to avoid having it all formally valued (which we would have to pay for of course)
Not necessarily. It depends upon what you mean by 'formally'. Most auction houses will give you a verbal valuation of items entirely free of charge. (Obviously, they're hoping that you'll go on to sell those items through them but there's no obligation to do so). It's only when you require a written valuation (e.g. for insurance purposes) that a fee is levied.
That applies not only to local auction houses but also to the 'big boys' too. For example, I've walked into Bonham's prestigious international auction house on New Bond Street in London (without an appointment) and asked to have some timepieces valued. Within minutes I was chatting to the same expert that people queue for hours to see on the Antiques Roadshow. He was happy to give me his time and a full appraisal of the items that I'd shown him. (Similarly, I've accompanied a friend into Sotheby's famous auction rooms, again without an appointment, to get an on-the-spot valuation of a vase. Once again, we were met by a famous face, who was happy to give of her time).
So seek out an auction house that offers jewellery for sale (either in specialist auctions or in general ones). You'll at least get a free valuation of the jewellery and you might end up selling it there, as selling by auction (where people are bidding against each other for your items) avoids any possibility of getting ripped off.