If it helps, I'm using a Pickit2 programmer ...whilst I've been away from all of this for a while, I don't recall having to do anything special previously wrt the programmer when using RA3 as a digital input (albeit probably using a different chip).

Just moved the Sitch off RA3...to another input pin, device is not resetting now whe the switch is pressed, so it is definitely related to when RA3 goes low, the 16f1824 is resetting!