Attenuation Tank Tedium

This for me is very very dull but I feel obliged to write about it briefly as it's been quite a big annoying issue for us;
During the design stage, planning and working with our code assessor no mention was ever made to us that we would need an attenuation tank to reduce the flow of rain water into the public sewers. It was not included in the plans, costed or quoted for but when our builder met with our code assessor it was brought up as being a mandatory requirement! Why it was never mentioned by our architect I don't think we'll ever know - during the design stage we had paid for a report to be done to calculate the rain water run off and apparently this tank is necessary for the one in one hundred year storm so that our house doesn't cause excessive strain on the the sewers.
The short of it is we now have to set a tank in the garden approx 4 metres long by 1metre deep. sounds simple but in fact it necessitates extra drains and quite serious digging, layers of various materials etc etc- yes I feel like going to sleep talking about it too!!!
We've been quoted £11k for this very dull thing - money I would have much preferred to spend elsewhere on something I can actually see and enjoy - as if we haven't sunk enough money in the ground already.

Due to the mammoth cost our builders are now trying to get quotes from specialist drainage companies to see if they can do it any cheaper - watch this space!!!!!