Running code on paid memberships
 

I'm looking for some advice

I want to write a very simple plugin/script that runs everytime someone pays for a certain paid membership

I have 2 paid memberships - one is £5 a year and is just for forum access, the other is £15 a year for club membership

When someone pays the club membership and their account is moved to the "club member" usergroup I'd like a script to run to assign them an auto-incrementing membership number

At the moment I have a script that I run nightly on a cron job that checks for everyone with a valid paid subscription who's membership number is NULL and assigns them a number - but I'd like it to be done instantly when they pay

Any help as to what hook I could use? or other way of doing this?