That Spectrum Audio page has "Discontinued" on it.

An update:

I called Line6 support and was asked to email them a completed form specifying what I wanted to buy.

(Note, Line6: If all your operatives do on the phone is tell me to email them, would it not have been simpler for everyone just to put the email address and the form on your website?)

A month passed with no word from line 6, so I sent a reminder.

Within two days I got a reply, and two brand new 50-04-180 jack plate assemblies were on their way to me, at a mere £16.56 each.

These are an improved part with the spring-loaded cover over the digital jack, no doubt because like me everybody wrecked them poking analogue plugs in there.

It went straight in the Variax 700 - didn't need the blue wiring harness, and I re-used the green earth harness from the guitar - and it all worked perfectly.

However, it didn't work in my original Variax (and early one before they renamed it the 500). - first of all the analogue jack is soldered in a couple of places, rather than using little plugs, so I had to do a bit of mixing and matching.

However, it looks like I'm going to have to bust the soldering kit out anyway, because when I plug it in via the digital cable I only get the signal in a square-wave-type on-off-on-off pattern. The new digital jack has a bank of surface-mount capacitors on it (as did the old one in the 700) whereas the original jack does not have these. I guess I'm therefore going to have to swap the small boards over on the digital jacks...

Oh well, at least I now have one working Variax, which I am delighted about. I will report progress with the other.