What type of agreement for sub-let of nearly self contained flat

Tenancy Types and Management

We rent a house on a standard Assured Shorthold Tenancy Agreement. The property has a flat above the garage, which we rent as an optional extra.

We decided to stop renting the flat recently, but the landlord would prefer to avoid this, and has given us permission to sub let the flat. He doesn't want to be involved in the let, he'll just amend our agreement to give permission.

We've found a prospective tenant and have agreed an ‘all in’ price.

What agreement should we use?

The important points for us are:
- We cannot give the tenant any terms exceeding my own agreement. On our own AST we’re now on the rolling two months notice from our landlord. We’re happy with this. We would like our sub tenant to have the same or less (ie one or two months notice).
- The flat is pretty much self contained – it is detached (above the garage), has its own front door, and exterior access (but via our garden). However, although it has its own boiler for heating it's on the same oil and electricity meter as the main house with no possibility to meter separately. Therefore the rent will include electricity/heating. It is fully furnished with a kitchen/lounge, bathroom and bedroom.
- There is no washing machine in the property, but there is the possibility to install one (landlord has a spare). With this and the metering could this influence the agreement type?

Could we use a basic room share/lodger agreement in this kind of set up or would it not be valid? We could potentially offer a service like cleaning to gain access. The tenant we have found is pretty flexible - the rent is set lower to reflect this.

Any other notes of caution?

Our main priority is to be 100% legal and not run into any issues with our own landlord.

Thanks in advance.
Scott