If you know for sure that only the Society knows this emails, there are 3 possible reasons:
(1) You have the email address listed on the web somewhere and the spammer has used an email harvester to collect your data.
(2) Email address was generated by an automatic tool which compiles milions of combinations of usernames and domains in the hope that there will be many real existing emails generated.
(3) RPSGB's database is compromised/sold
Thinking further, I suspect that the problem has been on the computer of the members of the local branch, rather than Lambeth. They have my rpsgb address as well, to notify me of local meetings, and are much more likely to have managed to get their computer compromised.
I shall be asking around at the next branch meeting.
As an alternative it may be that an email has been sent to a number of recipients including your address and then the address has been harvested from a comprimised machine belonging to one of the other recipients.