IHT returning forms by post

I've been trying to return a C4 form back to IHT, but though I've tried since July they haven't received it; despite using recorded, special delivery, and (at the prompting of the IHT team on the phone) by normal mail it hasn't arrived. Is there another address (apart from the BX9), as if the Post Office can't deliver to HMRC who can?? Can it - be done online? Also is there a problem/delay between scanning the documents (centrally) and getting HMRC staff to actually view and respond to the scanned documents? In addition to the 15 working days - almost a month - that's been suggested by HMRC IHT.

Hi,

Please contact the Inheritance Tax team for advice.

Inheritance Tax: general enquiries

Thank you.