Hi guys, quick bit of advice please, if anyone can?

We have a farm. We have our house and 2 further properties, both of which we rent out. One of our properties we rent to a couple of lads who are generally good lads. It's only a small property - ok for the lads, but they've started to have lots of people 'stay over' quite alot of the time. We've seen a marked increase in both water and electric costs, which we've mentioned to them, so they're aware that we keep a check on these things (for our records too when we get billed for the same). Both services are included within the rent and are stupid expensive at the moment.

My question is this. We have updated our draft lease document to say that only so many people may actually live in the property - I understand this is a standard clause. Would it be unreasonable then to add that no other people may stay without permission? Obviously we have the usual 'do not assign or sublet' clause in there, but this goes that one step further. We really don't want to be killjoys at all and we're not unreasonable people, but when we see them hiding people in the house so we don't see that they've stayed, it's a bit annoying. Especially too when we're trying to be conscious about the energy and water we use ourselves to keep costs down on the farm.

I've not seen such a standard clause anywhere. Does anyone else use one? We would never stop them having visitors at all, that would obviously be completely wrong, but just be reasonable about it and the amount of them that stay. We did have a friendly chat with them about it, but it seems to have gone in one ear and out the other!

Any advice? Thank you.