But, as said, they cannot supply the Vx sensor.
But they have quoted me a price to fit the one I supply............£295!!!.................but maybe cheaper if they take a shorter time doing it!
£295 for ten minutes work!
Five minutes if done to dealer standards
But if they supply a hookie sensor then they get to keep replacing it until it works long enough to last their liability period. 
Not to mention the time saving we spend smacking our heads against the wall...
Yes, thanks DG, I do understand that but no way is it going into their garage on the bassis of a quote like that! They cannot know what they are talking about, which is now common when it comes to some issues with our out of production, 'old' cars!

I can and will do the job myself once I get the sensor, one way or another

Go in there tomorrow morning. Ask to speak to the Workshop foreman. Show him the quote, then show him the guide, then ask the book time. Then ask them for a fresh quote.
It should halve the original quote as a minimum. The last Crank senosr I bought was around £78 inc. Add a the book time in labour and you should be around the £110-120 mark.
This removes all responsibility for the sensor supplied and warranties the part and labour for a year.
If it lasts a year, then happy days. If it doesn't, then you take it back and they fix it FOC.
In your shoes, this is what I would do and not worry about it.
Personally, in my shoes, I would seek out a genuine sensor from a genuine source. They're not made to order so there will be some in stock somewhere. That engine is used in no end of GM product.