Whether formal or informal all premises will have some sort of agreement in place, if not written it is implied. If not contracted out of the Landlord and Tenant Act 1954, it is likely that your lease will be afforded protection under the 54 Act unless otherwise specified. As such it can be contemplated that your landlord may wish to increase the rent at some point in time or bring the agreement or lease to an end.
With a lease in place, it will be specific on how and when the rent can be increased, and perhaps with an upward and downward it may go down. They may specify that the rent should be valued to Open Market Value, or it may be linked to an RPI increase.
At this point, the landlord may need to follow certain protocols to serve a rent review notice and it is in advance of this a tenant should seek professional assistance, to ensure timescale and protocols have been complied with.
Leases contain guidelines for the rent review, serving notices, agreeing on the rent, resolving disputes, responsibility for costs, completing the process, and paying a new rent. Generally, a rent review is to the open market rent. Market rent is what the premises would fetch if available to let on the valuation date.
The review process relies on a combination of the valuation guidelines in the lease and an opinion which includes the interpretation of evidence.
Every lease is different, every landlord, and tenant, and every property. And so is everything else. Comparable evidence doesn't just mean rent, it includes all the terms and conditions of the tenancy.
Valuation isn't only about what rents others have agreed: it also means comparing the type of premises, size, construction, use, layout, configuration, trading position.