In reply to Anonymous:
OP here... thanks for all the responses. I think my personal favourites are the vernier calipers in the night, the old money stuck down a tube and the getting the friend to try on the old aunt's rings.
However I think I have come up with the ultimate plan....on our next trip to a shopping/town centre I sneak ahead/away and run into a nearby jewellery store where I ask the friendly shop assistant to stand outside with a clip board and pretend to be running a competition to win a piece of jewellery, all we need is your ring size and email address - why not come inside for a free ring sizing?
Meanwhile I pretend to feign boredom/disinterest etc., I could even say I'll meet you later and surreptitiously get the friendly shop assistant to email me!
It's so shrewd I might apply for MI6. Who knew there would be so much deceit involved in this act of dedication!