We built an extension in 2014 for which we obtained full planning permission and building regulations sign off.
We've recently been told that we should have obtained build over consent for this from our sewer company because the extension has been built over a 'public sewer' (essentially our neighbours kitchen water, which pipes underground into our back garden where it meets our kitchen water, then they're both piped out to the mains sewer, via pipes under our extension). Never had a problem with any of these drains, they all work fine.
We had no idea that this consent was required at the time - we were led by the building regs guy from the local council, who seemed very thorough and asked for some extra structural calcs, plus other bits and pieces. But never once did he request to see the build over consent. He inspected our site numerous times and there was a whole inspection dedicated to drains and the re siting of the manhole.
We have spoken to our local sewer company who were very reasonable, said to ignore all the ridiculous "your extension will be pulled down to access the sewers" nonsense online, and to put in a retrospective application with a cheque for £400, and if no problems it should be granted within a couple of weeks.
But I'm slightly 
at how our extension got signed off by building control when the guy hadn't seen this piece of paperwork? I know it's ultimately our responsibility as homeowners to have obtained it (& if I'm honest I'm upset we missed this, I genuinely had never heard of it before), but how could he have issued a certificate if he hadn't seen this consent?
I wondered if anyone else has fallen foul (bad pun) of this, and if so - how was it resolved?