Thank you for all your responses.
To answer a few questions -
dp's dad is divorced, but is not dating or anything like that as he is a nervous wreck.
I am not sure it is normal for a man to look at other women 1 week before he gets married, I certainly have not had these kind of thoughts - if I did then I wouldn't be getting married.
By normal man things I mean not emptying bins, forgetting to flush the loo - porn is not in this relationship I made that clear at the start.
After speaking with dp this evening, I think I really do believe him that he hasn't done it, his family are pretty warped and we both have a feeling this may be down to one of his younger brothers who enjoy making trouble..
Not sure still, I am going to monitor things very carefully from now on