It's a 31BX1016/5S (a rare use of cm to express a screen size in a model number)
https://www.vintage-radio.net/forum/...d.php?t=154076
I've tried again with some different tapes & it seems to be OK on ones without macrovision.
Tuning on button 6 doesn't make any difference, I guess as the basic TX chassis design is from the mid-late 1970s there isn't anything special on the circuitry to sort this out, especially as macrovision didn't come around for a few years.
I know Maplins used to sell a "video smoothing" device that would correct reverse the effect of macrovision (not mentioned anywhere in the instructions!), but I'm not worried enough to track one down.
My other sets seem to be OK with it, my Toshiba certainly seemed to behave, not tried the Fidelity yet.