Ok, this has been resolved. For future reference, here is how it was fixed.
A bit of research on this forum and elsewhere led me to believe this was a pressure differential switch issue. So I bit the bullet and ordered one from here for under 30 quid delivered.
After much examination of the unit, I followed the directions in the installation manual on installing it which is roughly this:
NOTE - these are the instructions paraphrased - please get the instruction manual for the full instructions and safety precautions.
1) Switch boiler off and remove front cover and the cover which houses the burner (4 screws). Also, unscrew the lower cover which just tilts down (1 screw).
2) Close the CH inlet and outlet pipes. There is a little screw on them (you will need a short stubby screwdriver to get to it) - just turn them until they stop.
3) Empty the CH water from the boiler. There are two little valves for this which you unscrew, and the water comes streaming out. You will need a short piece of hose for this (I used a garden hose cutoff) and a bucket. It filled about half the bucket and took about 5 minutes. There are three seperate valves - two at the bottom at the inlet/outlet, and one at the top near the heat exchanger. Once the bottom two are empty (do them one at at time) put the hose back on the leftmost one, and open up the 3rd one at the top near the heat exchanger. Once this is open, it will release another quarter buck of water. Once this is done, close all those valves and you can continue replacing the valve
4) Replace the valve. There is a little clip at the back which just slides off - just pull to the left. Once this is done, remove the electrical terminals from the valve, and then just pull the valve towards you - it just slips off.
5) Put the new valve in place, attach the electrics, and slip the clip at the back on.
6) Switch the valves back- you will hear a rush of water into the boiler.
7) From here on just continue as if you were re-filling the CH after a flush.
Hope this helps someone in the future - if i've made a mistake or left something out, feel free to comment.