I had that same problem with my Mark V, same new clips easily broke.
I found I had to twist them a little bit so the two sides of the pinchers don’t interfere with each other as you try to pop the thing into the hole in the chassis; they have to cross each other like scissors.
I think I got mine from XKs Unlimited.
do you think the piece of tape is original?
in theory the spring clip should hold it firm enough not to rattle but it might be there to stop wear on outside of line?