Since it is just for added bling in the clock, You may want to think about building up a bezel ring around the clock face or other flat parts.
I don't know if this company ships to the UK, But it may be of interest for you to look and see what the have available in the silver, along with other base metals ,wire stamping etc.
metalliferous Bezel wires stamped flat wire stripmetalliferous silver home pageAll are PDF downloadable catalog pages, They do have an amazing amount of metals for modeling and jewelery and clock making. Or MadModdering, in addition to tools and other stuff you never knew you needed!
Here in the states hallmarking has a different meaning then in your country. In general hallmarking is the same as a Makers Mark In the US since there currently is no law in the US requiring a Quality mark. The current standard is the National Gold and Silver Marketing Act the standards is 921 parts per thousand or .915fineness with solder taken into account.
Plus if you quality stamp it it either has to have a registered trademark or makers mark, which if it doesn't (registered trademark or makers mark) the parties that produce the article is responsible, for producing fraudulently marked goods.
And as you know, Silver, Mexican Sliver,German Silver, Indian Silver or any other names or markings are no guarantee that there is any silver content in the article. Since German Silver is the old name for nickle silver, which has no silver content at all.
Silver from other western countries are marked with the parts per thousand numbers to denote the fineness of the silver content.
The US and the old British Empire countries use the sterling standard. All though your registration of hallmarks etc. is more rigidly controlled and followed. The hallmark site in the above link is the best site on the net for information on the matter.