Perhaps if instead of trying to get a small plastic air line with a fitting at either end replaced, I’d simply asked people to supply me with a carved unicorn horn with gold attachments, then I would have had greater success.
Eventually the extremely kind Joe, at NI Trucks cobbled one together from me using my fittings and pipe.
This done, I have air again, and the (rather tight and squeaky) brakes allowed me to move the truck. Hallelujah. This means that mechanically I’ve just a couple of minor things to sort now. Oh and the power steering sounds like its being tortured.