My ZX81 clearly didn't like being taken apart and the keyboard started failing once the ZXvid unit was installed, probably because the old ribbon cables had deteriorated, though they had looked OK.
I chose to get an enhanced ZXpand keyboard with the extra labels and did the swap in around 15 minutes. After dismantling the ZX81 and disconnecting the ribbon cables, the old keyboard peels off quite easily with the help of a hair dryer and something to slip under the membrane and tease it off.
The new one sticks on easily but make sure that it's properly aligned. I couldn't peel the backing off in situ so had a few practice goes and then went for it.
Re-assembled and all keys now work OK so finally I have a fully working, expanded ZX81 which I can use with a decent monitor.
I may choose a smaller one than this though.