UK Landlord Forum / Life

I have had private tenants now for 8 years, never increased the rent and now find that the rental value has massively changed and I am undercharging them about £300 a month
I know I cant increase the rent on renewal by more than 5% but what notice do I need to give them? Is it 6mths to advise that if they want to renew again the rent will increase by 5% ?
Also do I need to use a Form 4 to advise them of this change this form seems to reference periodic lease and this is not?
They are current half way through a 12mth ASSURED SHORTHOLD TENANCY AGREEMENT with a  fixed end date not a periodic agreement?