I once found £30 in the car park at Tesco and kept it. There was no point in going to customer services and having an announcement put over asking: "Has anyone in the store lost £30?" I would have been trampled to death in the stampede.

That is the extent of my luck in finding treasure. But for Mary Hannaby, things turned out a bit differently when she and her son, Michael, unearthed a 15th-Century gold pendant in a field near Hemel Hempstead one Sunday. Under British law, treasure is considered anything more than 300 years old and contains a substantial amount of gold or silver.

So what should you do if you strike gold?

If you find some treasure you should report the discovery to a coroner (strangely enough) and also to the museum that would ordinarily be interested in such a find. For Mrs Hannaby's find, it was the British Museum.

You should then be offered the market value of the item by the museum, which in this case was estimated to be around £4,000. It's up to you whether you take it or not. Oddly enough, although the British Museum said the pendant was "an important find" they couldn't actually lay their hands on the £4,000 to buy it, so Mrs Hannaby held onto it and went off to Sotheby's who valued the 2.8cm by 2.3cm pendant at around £250,000.

That, in treasure circles, is what you call a result.

Now, the small piece of jewellery, which depicts the Holy Trinity, is up for auction on July 9, and might even do better than expected after a similar piece, the Middleham Jewel, was sold at auction for £1.3million in 1986 and was later resold to the Yorkshire Museum for £2.5million.

So Mrs Hannaby, who might be considered slightly eccentric for her regular, six-hour Sunday forays into the fields with her metal detector stands to collect a nice little windfall whatever happens.

For me. it's back to the car park.