Does anyone know whether it is possible to disable the hardware reset button functionality and use it as an interrupt instead?
TIA
Not possible.
Are you wanting more interrupts ? If so look at pin change interrupts Arduino Playground - PinChangeInt
Reset is the ultimate NMI, surely? 
No I am not wanting more interrupts, just one. So rather than wire one in I thought that if the reset switch could be used it would be more convenient. More so because I haven't had the need to use it yet and as such it seems to be redundant.
However, if indeed it can't be done then I will have to wire one in instead.
You could reprogram the fuses on the chip via ICSP to assign reset pin as a generic I/O pin,
but then the bootloader wouldn't work.
Maybe
The Reset pin functionality can be disabled with fuses. The pin then becomes a digital I/O
Pin Change interrupt can be applied to another pin.
The end results is no traditional Reset, regained use of the physical reset pin, and some code to manipulate the new interrupt pin.
http://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html
Thanks for that, but as a newbie its beyond me! I will just hardwire a switch instead.