It seems this problem has come back again between UK and NL
I found this address for the Royal Mail on this forum and wrote to the guy:

Dear Allan Leighton



I writing as I am extremely frustrated at the service of parcel delivery between the U.K. and the Netherlands. I am currently waiting for two parcels to arrive ( both posted 3 weeks ago) from two different addresses in the UK. This is not the first time this has happened, in 2006 I experienced similar problems, with packages either not arriving or being returned to sender with the note that they were undeliverable. Indeed I found your address on a web forum discussing many preoblems people were having at that time with the service. It is highly frustrating as when one contcts the Dutch post they say that it is the fault of the Royal mail and when the senders approach the post office in the UK they are told that the problem lies at the Dutch end. I do not have any barcode references for these parcels; as there have been no big problems since 2006 it was thought that it was Ok to trust the standard service once more, clearly this was not the case.



I hope that you can pass my mail on to the relevant person or department and look forward to your reply



Yours sincerely