I'm using SD android (latest version). In the "airfield view" (the one that comes up after pressing on the "i") after selecting a runway the ILS appears at the bottom as normal but instead of a glideslope and localizer I get a glideslope and a heading bug. Once below 500' AGL and the glideslope disappears, at this point the localizer needle appears. The heading bug appears to be carrying out the functions of the localizer needle, so the thing is usable, but it would be more useful if it presented itself as a standard ILS.

Could you possibly post a screenshot? Does it say ILS in the back of the instrument or HSI at this point?

Tim it says ILS and with the correct runway. Screenshot coming up....

It won't do it in simulation mode, it's working normally! I think what might have caused the issue in real life was a last minute change of runway coupled with a fairly tight join. It looks as if when the needles are at full scale deflection they don't appear at all but are replaced with a yellow arrow? If so, I didn't realise but this would explain it.

Yes, an arrow indicates that the needle is deflected beyond where it can appear on the screen.