Thank you everyone for your help so far.

I think i may have found the problem causing the misfire! All the vacuum hoses looked to be in good condition, but i couldn't see part of the vacuum hose that goes to the centre diff lock, so to test it i disconnected the hose from the motor and plugged the hole. I have been to and from work twice now and it hasn't missed! (I bet it will start again tomorrow...)

If i blow into the hose from the motor i can hear a very quiet hiss further down the hose. When i get a bit of time i'll try to track down the leak. Hopefully its in the hose and not in the switch. Come to think of it now, the last time i engaged the centre diff lock i did notice that it took much longer than normal to engage.

Thanks again everyone.

Edward