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As the fixed term of the Tenancy has expired, you have a periodic Tenancy, meaning that the Landlord needs to give 2 months Notice, such Notice to expire the day before the monthly rent is paid to you if he wanted you to leave, and you would need to give the Landlord 1 month's notice in a similar fashion.
You may possibly find that the Buyer is an investor who would prefer a sititng Tenant, but obviously this can't be guaranteed.
Nearly every Tenancy Agreement will include a clause stipulating that the Tenant will allow viewings in the last 2 or 3 months of the Tenancy, and although your fixed term has expired, the terms of the written Tenancy Agreement continue under the terms of your periodic tenancy. You therefore need to get your Tenancy agreemet but I would be most surprised if it didn't contain this clause, I'm afraid.
I hope this helps you and sets out the legal position.