Just shows to go you - never rush to complete a message just because the lunch bell rang! Sorry I didn't check the denomination (since I had nothing to judge the size by), and glad you found the correct information.
No worries, I had no idea what I had to begin with. Your information pointed me in the direction I needed to go...so thanks!
Sovereigns from the 1930s and before have mintmarks from the various imperial mints, and as someone has pointed out, there were also the Kings Norton and Heaton mintmarks for bronze issues. I think the 'Soho' on Matthew Bolton's Cartwheel issues would probably count as mintmarks as well. The London mint no longer exists BTW, the last coin to be struck by the Royal Mint at it's old premises on Tower Hill was a 1974 Sovereign struck in 1975. Since then, every coin struck for circulation in Britain has been struck at the Royal Mint's new premises at Llantrisant in Wales. As for medieval issues, there were loads of different mints and mintmarks representing mints from as far afield as Durham in the North of England to Calais in Northern France, in addition to the London Mint based within the Tower.....