You're not the only one. You will see in this forum complaints from others about the "black hole" that items destined for HMRC at the BX9 postcode vanish into.
That postcode is not geographical, and it is not in the general RM database. It's an arrangement between HMRC and RM. Other carriers won;t even recognise it, they either won;t accept items addressed to it or will return to sender. HMRC give a separate address for tax returns and so on that are sent by couriers other than RM.
It seems tha pretty much all mail going to BX9 (the second part changes to reflect the tax department, for example, inheritance tax is BX9 1HT but self assessments are BX9 1AS) is processed through the NW Midland Mail Centre, regadless of where it comes from - you're in Kent and so is the HMRC facility, but your item still went to thr Midland first. Then it all goes to an HMRC facility in Bexley which does nothing but scan the documents for HMRC to process electronically. They do not actually process tax there.
The chances are that your item was delivered but RM simply did not track it. They get thousands per day, so it likely went into a sack along with all the rest.
Count yourself lucky it got to NW Midlands. I sent in IHT forms on 12th July via Next Day Guaranteed 1pm (and actually, Special Delivery is what HMRC themselves recommend). It got to my Mail Centre in Southampton on the 14th and nothing since. RM have been utterly useless. HMRC say they have no record of it arriving.
Note that HMRC do not record deliveries. Therefore, you have to wait the ten working days, or two calendar weeks, whch is the expected delay between mail arriving and being scanned into the system, before they will consider it 'lost' and instigate a search in their own facilities. But they do, afaik, record the RM tracking code as part of that.
What you CAN do is to reclaim the postage now. Since RM have no delivery trackign event, by their own systems, it is a failure of the Special Delivery guarantee.
Also make sure you keep your receipts from RM. If you get into a dispute with HMRC you at least have evidence that you did everything you could to ensure whatever it was arrived before the deadline. Note however that
delivery to HMRC is not the same as
arriving with HMRC. If it arrived on Wednesday this week, itmight not be scanned until Auguest 13th.
. Thanks again.