I have similar issues with HMRC, and have done for about the last six months or so. It took me three months to receive stamped IHT421s in cases where we either paid the tax immediately, or there was no tax to pay.
I also use the online portal to get IHT reference numbers (which you need to complete IHT423), and it is currently taking weeks to receive the requested reference number, despite receiving an almost instantaneous email confirming that the application has been received. Why can’t this email contain the reference number?
Taurean Drayak
Elliot, Bond & Banbury