Owain.. Back to basics. Fault is Mechanical, Fuel or Ignition. Go for the easy stuff first. So, your mechanical check rules out anything catastrophic. You have a spark, hopefully in the right place so you are right to suspect fuel. Before you rip it apart, try a spray with Easy Start, known otherwise as "Come on you ba****d.. If it fires up for a few seconds, it is more likely a fuel issue. No change, still won't go, then start with a check on the ignition. Try maggy cover off to eliminate unintentional grounding, check points gap, clean the slip ring, pick up brushes, check plug leads on right way round. Then check the timing. All OK? Then start on the fuel system.
Swarfy.