Just an update - I've been really quite busy with other things so my electronics hobby has taken a bit of a backseat ride; so recently I've taken the time pick this back up again. When I had the boards made I swapped out a couple of pins on the microcontroller in order to make it possible to use a larger chip without having to respin the board design; so a few weeks ago I decided to re-write the entire project using a larger chip with a view to including all the extras features I wanted at the time but had to remove, because I ran out of programming space on the chip, as it was only 8K.
My original choice of replacement was the 18f4550 but after a while coding I then discovered that although the USB feature of the chip can be disabled - which was my original idea to free up the pins, one pin in particular of the USB can only be used in one particular direction, so that made it incompatible without a messy board hack, so I turned to the 18f4685 with its 96K of code space and this looks like it might well work, and with its internal oscillator it will save me a couple of parts too.