A covert operation is usually always a good plan.
However providing they fix all your issues and they fit a genuine LUK DMF and a proper Exedy clutch kit with a Koyo release bearing then a £350 contribution is not unreasonable as they are wear and tear items and a degree of betterment will be evident.
However, I would make sure that the above parts are definitely fitted and would stand over the technician during the process.
The above all does sound a bit extreme, however you have painted a cynical picture in your post so needs must.
Only hand over your cash once you have test driven the car after the repairs, and you are entirely satisfied.