A trader who has requested anonymity has written from a village near Doncaster: "Following from your article 'Does everything have to go up?', I wondered if you could help me in my predicament?"

He took out a fully repairing lease five years ago (February 2004) for 20 years, with a rent review every four years. This spring he got a letter from the landlord's accountant saying that his client wished to increase the rent by £4,000 (a 20% increase from the original £21,000 pa). "We had suggested this amount seemed excessive due to the current economic conditions," he adds.

"After this we had a meeting with the landlord's accountant, who had agreed that his client's requests were a bit excessive and showed me an email that he had received from the landlord requesting a £3,000 increase and agreeing to £2,500.

"As you can see," he concludes, "I think the landlord is pulling any figure out of the hat that is going to benefit him. I was wondering if you or any readers would know what has been the average rent increase as a percentage so that we can resolve this matter."

I don't know if any such average exists, but I said I would ask readers. My files show that many factors affect rent values, including assessing floor space and an analysis of the lease. Rent reviews are based on the precise terms of the lease and the terms can have an increasing (and decreasing) effect. Clauses such as lease length, rent review pattern, repairing obligations, restrictions and whether a tenant can sublet all have bearing on the rental value. If you cannot agree with your landlord you can go to arbitration.

Meanwhile, I have urged him to take advantage of the Royal Institution of Chartered Surveyors' free helpline because if anyone knows the answer to the question, it's them (tel: 0870 3331600).