Is it worth adding a cai to my 1.2 500? Would I gain any improvement or would I be best to just replace the panel filter?

Thanks

Chances are you'd not feel anything, but placebo effects are not to be ruled out. GSR tend to have dyno figures which back up an improvement, but whether you'll actually sense anything is a moot point.

Not fussed about noise, would it improve mpg?

Technically, it shouldn't if colder air actually made it to the engine a richer mix would be needed which means more petrol.
Weirdly, after all my air intake mods my mpg has gone up,

Would replacing the stock box filter with k&n filter or such like make an improovment?

Maybe -- but we're talking a very little.

You'd save money long term over replacing the paper filters, though.

Ok thanks.