BC. Its bad news I am afraid. Until they declare they are cohabiting, there is nothing you can do about it.
Does he have his own house? Is he on the electoral role there? Is he paying utility bills there? If it yes to all these questions, you don't have a leg to stand on and will have to pay SM.
My experience is as follows: I found out about my wife's affair in July 06. I moved out of FMH in Nov 06 and have been living half a mile away in a rented house. I share care of children with ex2b on 50:50 basis, week on, week off. I have paid SM to ex2b for 11 months now and her boyfriend has been living in FMH "unofficially" since May/June. I have spoken to the Council Tax fraud team, as she is claiming single adult discount for council tax. Nothing they can do as he is supposed to be living with his Mum and Dad. He doesn't have his own place so he has no defence there but until they declare it, I'm stuck. I am keeping an eye on the new electoral role which comes out in December and will go and read it to see if he is on it the FMH.