I've learned this the hard way - how much Royal Mail, and by extension Parcelforce sucks ass. I've had several packages never delivered that have been returned because supposedlt no one was home to accept them - and today I had a note slipped saying I owe £15 in "customs" but the tracking number is a UK one so...
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Not that I'm expecting anything from abroad.
I was Head of Direct Mail and Media at the first company I ever worked for in London when I first got here - a company that happened to be one of the biggest mailers in the UK, so I had to deal with campaigns in the millions. The point of this is that I had to work very closely with the Royal Mail and there was not one day that I didn't feel like pulling out my hair when dealing with them. SUCH a difference compared to dealing with the USPS. Absolutely no professionalism and their MO was basically not giving a sh*t if they screwed up, which was every single campaign.
I read recently that there was an expose recently of mailmen here who basically THREW OUT mail without bothering to even deliver any of it, a lot of theft and a lot of can't be arsed.
You need to be persistent. You need to call whoever you need to, take names and make yourself a nuisance, or nothing will get resolved. I'm not saying be overtly confrontational, but be firm.