Tenants notice to quit

Tax Planning and Financial

My tenants have given my managing agent notice they wish to leave. There has been no problem with rent but I am suspicious that the notice period is considerably less than the required 1 month as stated in the contract.
The agent has not informed me of their leaving date, but informed me by letter, dated 21st May 07, that they will do an inventory check on 10th June. Assuming the letter was written as soon as notice was given and again assuming the flat will be empty on 10th Jun can I rightfully insist they pay up to the normal date of 25th June as the date of their notice to quit is unclear? I have asked the managing agent several times to send me a copy of the tenants written notice to quit to establish the date, without success (so far anyway!)
What is the legal position on this?
Would appreciate your feedback.
Regards
Peter Moran.