can tenant give keys away to third party?

[Bykergyal]

All I am a new poster, but from reading many of the posts for a while now, know you all are clever beans. I need some advice.

• Rented out my property 1st Jul 08.
• Tenant has been a problem ever since she moved in.
• Tenant stopped paying rent in April 09
• Served S21 April 09 for end of tenancy end June 09
• Served S8 May 09
  - Tenant called to 'negotiate' her leaving, as long as I signed a letter saying she no longer owed me any money and that I signed over the deposit (even though there were damages) - Obviously I did not accept
• Court Summonds to tenant sent June 3rd, for court date date early July 09

Property was origionally my home for 13 years.
I believed the tenant would 'do a runner' When she asked for her deposit back (even though she owed me £3K in rent). I guessed she had found a new place to live and would go before she got a CCJ or avoid a CCJ from not being found.
Last week, I called the upstairs neighbour to ask if he had seen or heard the tenant recently. He told me that he hadn't seen or heard her since the end of May this year (what I thought). He also informed me that the next door neighbours (whom she had made friends with) had been round to collect her mail, and told him she was 'away'. She has been 'away' now for three weeks
I asked him that the next time they came round, to have the tenant call him before he handed any further mail to them, for approval. She did telephone him.
Two days later (yesterday) the next door neighbours returned (the tenant had given them the keys) to enter the property and collect the mail themselves.
My upstairs neighbour informed me of this as the next door neighbour could not get in, because the lock sticks, and rang his bell for assistance.
I believe my tenant HAS abandoned the property, but as she has not given me back the keys is technically still responsible for the rent.
The questions I have are:
Has the tenant the right to hand their keys over to a third party to enter the property (and the common parts of the property), without consent from the landlord - there is no mention of this in the TA?
If no, I would ask the next door neighbour to either hand the keys back to me or refrain from entering the property. If the neighbour gave me the keys back, could this be construed as illegal eviction, even if she has a second set of keys and the locks were not changed?
[I]I am aware the tenant is very shrewd and has employed a solicitor to maybe lure me into making a mistake.
Lastly - The court summonds was in mail picked up by the next door neighbours, would you advise I continue with the hearing, as if the summonds has been served as normal. - or would you advise, I look about adjourning the hearing, until I locate the tenant in her new home - possibility that she could be staying with friends, and so I will never find her!
I am sure the neigbours are just trying 'to help' and are not aware that they may be embroilling themselves in this mess.
However before I 'get in there', I need to be sure, so any advice would be grateful.
Bykergyal