Thames Water sent some third-party guys out to find out the cause of the blockage. These chaps were very generous and they decided to un-block the pipes even though it wasn't Thames Water's issue. .
So a third party sub contractor knows what TW has or hasn`t adopted - I`ll bet not Reminds me of SEWater supply guys @ a M8`s farm - stopcock in road was "missing" - took about 6 blokes and a mini digger to NOT find it - One old P*key plumber from The Forest found it ,and the run to the house . Yes folks , the wonders of Privatised Utilities
Thanks Nigel. You've got a point. They came and fixed it without a fuss so you're right it must be there responsibility. If and when it happens again, I'll call them and hopefully sooner rather than later they'll dig it up and fix it for good.
I'd keep calling TW for as long as they'll come out. Repeated calls to same address should start alarm bells ringing, there's clearly a problem down there. Really needs a CCTV survey to ascertain what is going on and take matters from there.
If TW will fix for free all well and good, but it might be worth checking your home insurance NOW to see if this would be covered in case it falls to you to sort it out. (May not be cheap! )
Good point Hugh - but I`m still convinced the problem is in the shared / adopted part of the system, downstream of the manhole @ the front of the property - it`s then backing up into the OP`s drains . It`s the same logic I applied to my M8`s farm water supply
Hugh and Nigel thanks for your responses.
Good news is TW turned up yesterday and fixed the issue. Now they're going to do a video survey "free of charge" to work out the cause of the blockage and they also want to know if other properties are linked with this drainage. I'm hoping they come and do it soon because it gets blocked again.
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Reminds me of SEWater supply guys @ a M8`s farm - stopcock in road was "missing" - took about 6 blokes and a mini digger to NOT find it - One old P*key plumber from The Forest found it ,and the run to the house 
