Thanks - I have asked a solicitor friend of mine, and as long as we werent working there at the same time then he thinks we shoud be fine. I will try AW.
It's still a 'brothel' even if you're not working at the same time and that's something which is illegal to own, manage or allow to remain in premises you control... at least two of which you'd be doing.
These offences are separate from the 'controlling for gain' one. There may be an element of that too - you're telling someone when they can work (only when you're not using it), and gaining from their work (the rent). Your lease may well have something to say about this too.
It's unlikely anyone will be fussed, but this depends to an annoying extent on where you are. Some police forces (hello Tim) go after this sort of arrangement, while most others quite reasonably think they have much better things to do with their time.
As ever, don't annoy the neighbours.