Thanks for the tips guys. I decided I'd try and create a similar grove in the shaft by using a small needle nose pliers, and squeezing that with a vice pliers set to the appropriate gap. Worked well, and they sit very nicely, feel tight to a pull test (un-modified ones come out with the slightest of tugging) and survived a couple of games. Unfortunately one must have been damaged in the process, so now I'm down another one, and still not finished this area!
Overall, it's a big thumbs down to the #555 leds unless you want to fiddle around with some of the fixes mentioned, or source the Pinball Life ones which seem better, although some RGPing suggested only marginally overall. Why **none** of the manufacturers took three seconds to look at the design of a 555 lamp and mould the same is beyond me.
