Tony,
When you say 'ignition switch' do you mean the whole shebang, as in the key, barrel, housing etc.....
....or (as I suspect), do you mean 'just' the plastic lump which screws to the end of said gubbins above, and rotates with the key, making a variety of ignition connections at each point?
I 'think' it's the Cavalier Mk3 part, but it may be a different one - easy enough to check by looking at the back of yours, but they are cheap and availlable
https://www.ebay.co.uk/itm/311468047183Having thought about it - far more likely to be the Cavalier Mk2 one .......
https://www.ebay.co.uk/itm/202103992068Either way, it is a 'standard' Vauxhall part. Lotus did have the whole thing on the aftersales list a few years ago and I bought one as it came with 2 keys, and was cheaper than getting a spare key cut to the one I had!
Often, the rotating ring in the 'black box' gets corroded, and doesn't make the right collection of connections for its relevant position, or (in your case) travel quite far enough to make to starter motor connection. It may be worth giving it a good squirt of contact cleaner, as it appears to be an intermittent (but getting worse) fault.
If you need any further advice on how to get to the wiring and swap the bits over, I'm sure that if you asked one of your customers (preferably one who would have been a teenager back in the '90s) - they all used to carry a 'black box' and screwdriver to get home in a Cavalier SRi
