"Dear David,
Thank you for your email. Regrettably, we must refer you back to your place of purchase with regards to these wagons. The pack you have is one of a number of SKUs which did not enter the market legitimately, and thus we are sadly unable to offer product support on it I'm afraid. There is an intention to include the coupling pack among available spares in the future."
This was after I'd said I bought the pack new on eBay and that I'd seen other issues with this pack from other listings on eBay and elsewhere. Looks like a legitimate product to me, not sure how I'm supposed to check that anyway.
Pretty pathetic yet completely predictable response from Hornsby's customer service, if you can call it that.
To be clear my only ask was if they could send out some of the couplings, which must cost about 1p to produce, nothing more.