I assume therefore that youve been issued a s21 notice (notice to vacate the property) and that this is dated and filled out correctly?
The fact you haven't got an inventory might actually work in your favour come the time you leave the property. Without an inventory the landlord cannot make any deductions from your deposit for dilapidations etc.
Did you actually sign a contract? If you havent, again it shouldnt matter as you will have an contract by default with the landlord if youve been paying rent etc all this time.
You dont have to deal with anyone apart from the landlord or his agents. The agents can give keys to who they want but no one is allowed into the property during your tenancy without your agreement. This goes for the landlord, the letting agent, estate agents, prospective purchasers etc. You dont have to agree to people viewing the property even if it says you do in your contract (it would be an unfair term). You have the right to 'peaceful enjoyment'.
Hope this helps.