Rules re Sale & Rent Back (SARB)

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Would be grateful for some advice, pls.
We are hoping to buy a house from an owner who wants to remain as our tenant - i.e. a SARB. We have no other similarly tenanted properties, so I understand that this is OK - but I read somewhere that the tenancy agreement must be for 5 yrs - whereas an Assured Shorthold Tenancy can only be for a max of 3 yrs!!
Where does this leave us? Can anyone clarify?
And are there any particular pitfalls which we should be aware of? Your help wld be much appreciated!