Job done, slightly disappointed in that the units that arrived were black, but shiny, but they were cheaper!
Found an old broom handle that was just the right length to prop the boot while working; it now opens and closes as it should.
I couldn't figure out how to undo the spring clips on the old ones at first but a small electrical screwdriver levered them out far enough.
One unit was very soft, the other definitely below par. Oddly the bonnet ones are still fine, but I guess they get less use.
The replacements just pushed straight on once I'd managed to compress them slightly to fit, better to clip the bottom on first.
Discussion with eBay to follow - not as described!