Rent increase

Other

Hi we have some tenants in one of our flats for about 3 years and have not increased the rent up to now, but now we want to increase it. Is there a percentage increase that is acceptable? We know we could get around £80-£100 a month more than we currently get but they have been good tenants. It was initially a 6 month tenancy AST (one of the RLA forms) which has just rolled on. Do we need to use a particular form to do this or just write to them or do we need to have them sign a new tenancy agreement? Do we need to give them a particular period of notice before increasing the rent?

Thanks for your help.