Rent increase

Tax Planning and Financial

My current tenants signed a new agreement at the beginning of April supposedly for 6months, which we agreed verbally, but I've mistakenly made it for a year without question from them. A month later I contacted them because I did not receive the re-signing charge of £75 from them which is part of the least agreement. They said that it is not due as they are signing a new lease with a different 3rd person and therefore it is not a re-signing. I now realised that the 6 months expired at the end of September. I contacted the head tenant and asked him if they are staying on and he said that they would like to. I want to increase their rent as I have not done so since they moved in 2 years ago. What could I do if they said that their lease agreement is for 1 year?

Cheers,