So, got the new one fitted and put unused spare on front as well.
Asda, well, there's got to be a catch hasnt there........but I can't find it! Tyre was new in week 32 of 20, so only a few months old, not old stock etc, and best price i couid find.
Guys at tyre company measured depth, 12mm on new and 7mm on rear (so 50,000miles on 5mm of wear!), which is outside of Toyota max 4mm difference, but in a clever Baldrick style plot, I bumped the rear pressure up to 40psi and kept front at 35psi (normally 36/38) which has increased rear tyre height by 3mm overall, this gets the difference down from 5mm to 3.5mm and prevents any wind up - noticeable on reversing on lock.
Next is to get the front steering alignment set up properly again.