I'm so sorry to hear you've been affected.
I'm in the same position here in Merton, London. We submitted the exemption form late, after we started the extension. We received no information on the topic (the letter they posted wasn't delivered, agent didn't forward us the email) plus their CIL officer said 'residents are exempt' but never said anything about the form needing to be approved at a certain time point, so we thought it was a like a gas/ electrical sign off certificate. They are hitting us with an £86k bill with no appeal whatsoever.
We are not alone- I'm in a Whatsapp group with many homeowners and between us we've identified around 75 families across 12 different councils who have been hit in this way, more are joining ever week due to increasing press coverage and parliamentary pressure.
May I suggest you connect and we can see how we can support? Please feel free to message me individually, have a look at the Facebook group (Fight CIL Injustice), Google 'CIL PennyPost/ Maria Dobson from West Berkshire [who you may have seen refunded unfairly charged residents] or Steve Dally from Waverley- we are all in touch.
Lastly there is also a petition that was started recently: https://www.change.org/p/stop-the-community-infrastructure-levy-injustice-protect-self-builders-and-homeowners
A lot is a lot happening on this very topic and the more of us who share our experiences, the stronger our voice will be.
Please everyone please feel free re-share this post, or suggest where else this can be posted.