Once again, thanks for the assistance here Howard
I found version 2.1 of Johnny5 sitting on my main PC and decided to give it a go.
Lo and behold, I didn't get the run-time error when bringing up info on my test rom (chplftbl). It displayed the controls fine, except that instead of displaying the game name (I use your default lof file modified to show my panel and using my key mappings for Mame) it displays all dashes (ie: ----------------------). I have attached a screendump. Again, it only appears to be clones that don't display the game name correctly.
Any idea on this one? As the debug also seems to work in this version, I have included the debug log.
So, I'm not sure what has happened in 2.5 to break things for me, but at least I now feel as though I am making progress to getting this working. Now to get the game name displaying correctly...