Here's an update. Ordered a new part. The original part number is ACU5037. Struggled to find an original part from the usual suppliers so order the superseded part number AMR6676. However, the diameter of this newer fitting is a tad larger than the original. Did some more digging and found somebody who could source me the original part. It was for an MG Rover. However, the part number is the same, ordered it and once arrived it is indeed the same. Some sharing of parts between factories. Yippee!
Top one is the new ACU5037, bottom is the original. The middle one is the AMR6676
Once the new part was refitted, with the arrow pointing towards the rear screen, screen wash pumped from the reservoir to the rear window. All worked well, no leaks.
This is likely to be the first time in nearly 19 years since new that the rear wash has been working.