Hi Tom,
Thanks for that; I should have put it on the original message..doh!
The mark looks a little like a K and P but it looks like the P is crossing one of the slants to the K (the one at the bottom half of the P. It's really hard to describe sadly and the photo isn't that clear as the mark is so very tiny; looking at it again it looks like the P is slanted across the K to the right??
Sorry I can't help much! It does appear to be very old though in design. I might be able to get a better photo tomorrow in the daylight.. that is if the sun actually shines!