Essential works for sitting tenants

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I have sitting tenants on a regulated tenancy, in a two-bedroom flat. They have been in occupation for over 50 years and are retired. The new regulations for Electrical Testing has highlighted that the flat needs to be fully-rewired, together with essential work for fire alarm etc.
Unfortunately the tenant have a lot of possessions, making it impossible to carry out such work unless the flat is emptied. The flat is extremely cluttered, a situation borderline on hoarding, but there is nothing unsanitary to flag up.
I am willing to offer them alternative temporary accommodation, an immediately adjacent 2-bedroom flat, while works are carried out and for storage of their possessions.
If they agree to such an arrangement, what sort of tenancy agreement should I give them; can this be done by a written license to occupy? and can I expect them to accept responsibility for all utility bills, on the temporary accommodation. I would expect they continue to pay their current monthly rent, but not additional rent on the temporary accommodation. The tenants desire to move back into their flat once works are completed.
I may take the opportunity to also install central heating, as this is something that they have requested but has not been possible due to the crowded circumstances, subject to their agreement to a rent review/re-assessment.
All previous landlord obligations for repairs, Gas Safely certificates etc. have been met in the past.
Are there any other points that need to be considered?