Before I posted my question I was watching a pair for either £20 or £25.
Posted the question, looked again, no longer for sale......but there are others, with widely varying prices.
...and on the subject of heated windscreens, I phoned my local Auto Windscreens today to see if they'd swap my old screen for my heated one. Yes, they'd do it. But they would have charged me around £200 for the job.
I helped my Dad remove and replace a windscreen in his first Mini (he bought a 1959 one around 1971 and stripped it down to a bare shell, cut out and patched everything that was rusty, and gave it a complete respray). Hard work but not impossible.
How hard is it to remove/replace a screen in a Midas?