Can you collect it from the ParcelForce depot/local DSO? They may also have an option to collect it from a local Post Office (usually a charge of 50p or summit).
When I first moved in to place, the builders had literally finished the day previous (2 months late) and it too wasn't on the Royal Mail address database, but when I was requesting deliveries, I found all the delivery services to be quite accommodating when explaining how to find the property. When Royal Mail initially added it to their database they added the plot number instead of the apartment number, and I still get the odd item of mail coming for the plot number rather than the apartment number for some reason, which is a bit of a worry as there is a slight discrepancy thanks to there being no number 13 lol - fortunately the Post Man seems to check the name/realize it says Plot.
After 5 months though, your address should deffo be present and correct on the Royal Mail database, I seem to recall when I queried it being a plot number which was incorrect, they said it take a maximum of 6 - 8 weeks to update, though is usually much quicker than that. Have you tried speaking with ParcelForce customer services and working towards a resolution with them?