Using a letting agent can be beneficial as they are able to look after the receipt of the rent, deal with ad hoc tenant queries, get essential maintenance work done using their own network of tradesman and carry out regular inspections of the property.
Check what's included in the service and what's at an additional cost by reading the agent's contract carefully.
Most Assured Shorthold Tenancies are six months in duration, though 12 month tenancies are common too.
Providing the terms of the tenancy have not been breached, you cannot regain possession until after six months (or longer if the 'fixed-term' is longer) unless the tenant agrees.
So, for example, if the period of the tenancy was from 1st to the 30th day of the month and you gave notice on 10th August, the period of notice would not actually start until 1st September and would end after 30th October.
Your local Rightmove member agent should be able to advise you how to end or extend a tenancy and the correct form of wording to use. They may be able to arrange this on your behalf although there may be an additional charge for this service.