Has anybody tried these? I bought some and just tried them out, but they don't appear to flash as brightly as the normal orange ones. Obviously I'm concerned about safety if they don't shine as brightly as orange ones - but maybe just that I bought the cheaper ones off ebay, instead of the more expensive ones!

It says that they're road legal though...

Not sure if brightness is a legal requirement. I know they have to flash orange.
My wife brought some cheap head light bulbs from Ebay. They were meant to be very powerful and have a crisp white light. They were $#!+
I wouldn't take "road legel" on Ebay for granted. Take it to an MOT test centre and ask their advice.