I am in the process of overhauling the two cylinder heads on my Rover 75 connoisseur. All was well until it came to replacing the collets. I went to youtube after 2or 3 frustrating hours. Here is the solution and it really does work. I found that because the Rover's collets are so tiny they insisted on adhering to the plastic that the author of this video uses. I used a piece of duct tape folded over so that the adhesive side is on the inside. Even at that the collets tended to stick to the duct tape but the solution is to place a piece of polythene from a freezer bag between the duct tape and the valve spring retainer cap. It works because as you compress the spring the plastic stretches over the valve stem and as it does it presses the collets agains the valve stem. This really does work.
The link is:
https://www.youtube.com/watch?v=xtO8ev0Q9jc
Fred