Thanks for your reply.
You've got to get back to him in writing (preferably by email) attaching a copy of the tenancy agreement and referring to the above clause. State that you refuse his request to permit viewing any earlier than 17th July (ie. when the last two months start).
If there is a bolt on the door then you are entitled to lock it.
State that any entry earlier than the above time and without your consent would be a breach of contract and you would be minded to sue him for any financial loss you suffer as a result and inconvience cause. In reality you cannot sue for inconvience because it is difficult to attach a financial amount to. Refer him to http://www.moneyclaim.gov.uk/, this is the governments money claim service. Tell him it's very easy to issue a claim and you will have no qualms in doing so if he breaches.
The practical reality is that if he does visit then although it is a breach there is not a great deal you can expect in suing him unless you suffer financial loss, but he may not know that.
I would word the letter tersely but stating that you are happy to agree to viewings within the time period afforded in the contract and would be more than willing to negotiate convenient times.
You can also state in the letter that if he accesses in breach then you shall attach a bolt to deny him access the next time.
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