thank you. I note the actions you have taken and the information you have previously been given on the matter. I do not disagree with my colleagues advice however I am concerned that you appreciate that it is not easy to surrender a tenancy unilaterally and is certainly not an absolute right. The tenancy can be surrendered with the agreement of the landlord, under any contractual right you may have or if you can demonstrate that the landlord is in breach of contract on a funamental term.
What this means in this case is that you would need to demonstrate that the property is uninhabitable as a consequence of the plumbing issues. This is as you will likely recognise relatively high hurdle. A smell of sewage is unlikely to be sufficient to give rise to a surrender of Tennessee unless the smell is so overwhelming that it is not reasonably possible to live in the property or if it represents a health hazard. The difficulty with smells as they are hard to evidence as he cannot take a picture of them. Rather, you are limited to describing them which is subject to dispute.
The position as things stand is that you have asked to surrender the tenancy and the landlord through his agent has remained silent on the point. Until such time as the landlord accepts the surrender, the landlord can claim that a tenancy continues to exist and you owe rent under the tenancy end of the landlord refuses to accept a surrender, then you may need to consider an application to court for a court order as regards surrender.
There are various options available to you. the key would be to continue to impress upon the agent your requirement for a surrender and if they refuse to engage, you may need to consider collecting some evidence to support your position that the property is uninhabitable as result of the stench. To do so, you can either contact the local authority's environmental health department and asked them to inspect the property as a health hazard or under the housing health and safety rating system inspection framework.
You can consider contacting the agents and precedent for a response to your request and if they refuse to accept a surrender, advising that you therefore request the keys in order that you can organise a HHSRS inspection and plumber to provide reports in respect to the drainage and that you will be seeking damages and an order that the property is uninhabitable as a consequence of which the landlord is in breach of contract. If they refuse to return the keys to you, if you can document this with evidence such as a video recording or telephone recording, this would be evidence in itself of acceptance of surrender because on the one hand they cannot claim the rent from you and on the other refuse to give you keys.
If you need to resolve this matter in the County Court, you can do so using form N1 seeking an order of breach of contract on the grounds that the property is uninhabitable there as above, you will need to consider having supporting evidence if such a claim were to be brought.