we appear to have an extra person living at our property, but have difficulty proving it, what next?

General Discussion

We suspect that the tenants have taken in an extra person into our property, but have difficulty proving this without a flash inspection, or risking appearing to harass by calling round and confronting the tenants on the doorstep. It looks as if this could blow up into a bad situation, as they have lied previously, denying any knowledge about an extra car that was regularly parked outside the property that is now being parked on the path.
We now find that a similar car is being dismantled on the driveway.
Any help or advice appreciated as we are new landlords