Insurance when Tenant has a lodger

I let my 2 bed property to a single tenant. She wanted to take in a lodger in order to help her to afford the bills. I agreed and helped her to find a suitable lodger.

My understanding is that the lodger has no rights to remain in the property if the tenant or myself wish her to leave. We drew up an agreement between the tenant and lodger that clearly states that either of them only has to give 2 weeks notice for the lodger to leave. They share the house but each have their own bedroom. However, the tenant has stated to the lodger that she needs access to the lodger’s room because the water tank and boiler controls are in there. The room does not have a lock on and the lodger cannot refuse to allow the tenant access. 

The problem I have is that my Landlord’s Insurance is now invalid according to the Insurance Company. They refuse to provide insurance if my tenant has a lodger. They seem to be of the opinion that the lodger is a subtenant.

Is this common practice with Insurance companies? Does anyone know where I can get Landlord Insurance that will provide cover for my tenant and her lodger?

Thanks

Debi