I have a ladies W1312 I purchased from new in Dec 1999. Much loved and daily worn and stupidly took to a local jewellers and last night whilst playing sport the back of the watch has become loose and fallen off. Watch is still working but not properly as insides exposed! I am desperate to find a replacement or alternative solution as been part of my life for so long and hoped to hand down to my eldest daughter

 

Did you mean the WK1312? I can't find a W1312.



I don't know of an easy solution other than trying to find the caseback that fell off. You might try contacting TAG Heuer Service and see if they can provide a replacement part. But if they can, I fear it might not be that cheap.

You could also constantly scan eBay with the hopes that someone is selling a used one cheap or try to win an auction with a very low bid. Then take the caseback for your own watch.

 

I did a quick search on eBay for "tag heuer 2000 caseback" and there are a few for sale. If you measure the diameter on your caseback hole, you might find one that matches.

Also, this site has the caseback for the WK1320, which I believe is the two tone silver & gold version of the WK1312. So this caseback will probably work for your watch, assuming it's the WK1312.
https://www.chrono-shop.net/en/fond...-ssteel-caseback-professional-200m-sport.html