Hopefully you will appreciate that the following information was not the result of all that much detective work by me but I did find the owner’s association website (https://www.snapdragonmirage.org.uk). 
In 2019 the old website received a major makeover and you don’t have to be a member to see a potted history of the company and their products. Joining gave me access to a selection of scanned design drawings (low resolution so dont plan on building a boat from the lines drawings) and links to other owners’ blogs with information on the individual boats. That alone was worth the £10 fee. You will see that this was key to working out the type of boat I had bought because it wasn’t actually as advertised.
The relevant history from my perspective is that Thames Marine was formed in 1964 when Thames Structural Plastics moved to Charfleet on Canvey Island. The snapdragon 23 centerboard yacht had previously been produced in Southend. The move allowed them to expand the range and several models between 21 and 29 feet in length appeared in the next 10 years. Those included the Snapdragon 21 (20′ 10″ LOA with bilge keels & skeg hung rudder) and the Snapdragon 670 (6.7 M LOA bilge keels and transom hung rudder). To me these seem to be essentially the same hull with the LOA being altered by the transom hung rudder. To add to the slight confusion, the Snapdragon 670 design was also licensed to another company which sold it as the Mirage 22 (6.70 M = 21’11.78″)- Hence the appearance of multiple different boat names in the same owner’s association.
It seems that some of the boats were provided as fully finished and others were part completed kits for the buyer to complete. There are no obvious clues which category my boat fell into, I dont know the sail number and I have not been able to locate a manufacturer’s plate in the 
location where the Snapdragon 23 information suggests it might be. There is a nice carved plate just by the galley though so maybe that is the same information. – Maybe those are signs that she was a user completed version ?
The Snapdragon 23 received somewhat of an unexpected boost in popularity from 2011 to 2014 when Practical Boat Owner (https://www.pbo.co.uk) bought one from a farmer’s field and 
transformed her back into a smart family cruiser while publishing articles about each stage. Maybe that at least partially explains the error in boat identification in the advert. Not so bad though because at least I had the restoration project handbook to inspire and help with some aspects of my project. It took them 3 years so maybe I haven’t been all that slow.