So I started to look into adding the panel functionality, and what do you know - its working already; its simply drawing to the wrong screen! The plus4 version sits the panel on screen 2, but on the C64 one I'd used screen 1...simple. However while fixing this, I happened to spot another bug and one thats again in the Plus/4 version.
The panel is only supposed to get updated when it needs to, in fact theres a set of flags set aside to enable updates of certain components of the panel. This means that when nothings happening, its all very quick. However, the flags don't appear to be getting set and the only reason it was working, is because they weren't getting cleared either! So, I've now got to go and set the flags whenever the panel needs changing (like adding to the score, losing a life etc...). Pretty simple fixes, but one that means Im not spending time doing stuff I simply dont have to do!
On the plus side, the panel code is unchanged from the Plus4 to the C64, which means I can fix it on the Plus/4, then simply copy it over and have a working C4 one too... cool :)