Originally Posted by Miauuu

The new pipe is a one piece molded part vs the original design two part ultrasonic welded design which was separating at the seems. No need for metal.

Originally Posted by OzXFR

Yep, that worked.
Topped the expansion tank right up, started up the car, wiggled the suspect hose/pipe/tube a little, and suddenly coolant started pissing out of the right hand end.
Shut it down, cleaned up the area, and immediately saw that the outer edge of the clamp on the right hand end of that tube had cut into the tube and caused it to split about one third the way around the circumference.
It's a clamp very much like this: 8mm-10mm Mini Hose Clips Stainless Steel - Fuel Pipe Tube Clamps x10 | eBay
Obviously not a standard part, it's just what the mechanic used to attach each end of the MacGuyvered replacement tube.
It was a bear to get that clamp off, but once off I could see the full extent of the split. I've bodged up a temporary repair for now with silicon sealant and electrical tape, but now I can't get that clamp back on anywhere near tight enough, so I'm off down to the auto parts store to get a replacement clamp, this time a "normal" hose clamp.

At best that will still result in a temporary fix until I can get a proper replacement tube, but for the life of me I can't figure out which part number it is.

Well at least there is some good news - it should be a cheap and simple fix and not a messy expensive one.