Trying to sell, tenants making things difficult!

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I informed my tenants almost 3 months ago that I was intending to market the property that they currently rent from me, however if they wished to stay I would ensure that the property go to an investor who agrees to honour their contract.

Now a month into their latest 6 month AST, they have decided they wish to move out, they stated a number of reasons, all of which are particuarly petty, however as I am not an unreasonable person I have verbally agreed that should they provide me with 30days written notice (which I have not yet recived) and comply with the estate agents who are organising viewings, I will release them from their contract early.

They are now refusing to answer the phone to myself or the agents!!

Does anyone have any advice to offer on the situation?

Thanks in advance!!