increase in rent

I have had the same tennant for two years and took a bond before the new legislation came in force. I am wanting to increase the rent but am unsure if I need to issue a new tenancy agreement for this. If a new tenancy agreement is needed, will I now have to protect the bond in one of the schemes?

I might add as the tennant is in receipt of housing benefit which is paid direct to her two weekly in arrears, I agreed that I would receive the rent payable two weeks in arrears. The inital bond taken was the equivelant of two weeks rent.

The inital AST was for 6 months which I left to go onto a rolling contract.

Can anyone advise please?