The following article appeared in the Landlord Law Blog.
If your tenant stays on after the end of the fixed term, you do not need to give a new tenancy agreement unless you want to.
Assuming your tenant is an assured shorthold tenancy (which most tenancies will be in England & Wales), under s5 of the Housing Act 1988, immediately after the fixed term ends a new ‘periodic’ tenancy will be created automatically.
The ‘period’ will relate to the rent being paid – so if the rent is paid monthly it will be a monthly periodic tenancy, if the tenant pays weekly it will be a weekly periodic tenancy etc.
The tenancy will be on the same terms and conditions as the preceding fixed term tenancy, will be between the same parties and with the same rent.
So you only need to get the tenant to sign a new document if one of these is going to change – for example if one of the tenants is being replaced with another or if you want to increase the rent.
However, note that for the time being you will need to re-serve the prescribed information, as a result of the Super strike case, until such time as the government amendments to the law, which I understand are under way, come into force.