Hi Rhodri,
It looks to me that possibly it is the mark (upside down) of Jeremy Johnson, but it would be good to hear the thoughts of others to confirm/deny, especially
agphile as this is right up his street.
Should it turn out to be Johnson, then he came from Chislehurst and was apprenticed to Edward Hole in 1640. He is thought to have died in 1675.
Trev.