When you let the property, the price is for the property and not per person so I don`t think he can charge more. He is looking at the situation from a financial point of view but that aside, I can understand him not being happy with persons unknown moving in. When you let out a flat you choose who moves in and you obtain credit references etc and want to know something...
Tell them to leave - the leaseholder has no right to sublet without your permission (but check the lease first, there should have been a clause in it emphasising this)
And if it is a tenancy agreement (or indeed a lease) did it specify who was allowed to live in the property apart from the tenant?
"...but instead lives with a couple in the flat." seems to indicate that he is still living there but has people staying with him. This may not meet the definition of "sub let".
it was not agreed the tenant allow others to share the flat with him. Others might be paying him rent that llord doesnt receive; isnt that sub-letting?
No...sub letting is when the tenant moves out and rents it to someone else. If he has someone living with him they are probably paying their share of the rent.
ummm has highlighted the problems you may face. Even if the tenancy agreement (or lease) stipulates who or how many people may live in the proerty you may face great difficulty having it enforced if the person who signed the agreement still lives there. Of course there are issues of overcrowding to be considered but if this is not a problem you are effectively saying who the tenant may or may not invite to stay in his home. You need to see a solicitor.