Volkswagen: 1972 type 2 fuel injection, inductive timing light, robin mills

Question
Hi Rick, Sorry for delay, things seem to be getting worse, I   have renewed
the temp sensor as you said, i also replaced the fuel filter, now the engine
will not start up, it trys to start and will run for 1/2 sec and then just cuts
out, petrol pressure is good and is reaching all cylinders, the ignition is all
good with good spark at plugs, as is timing, i removed the distributor to
check the cb gap, i put it back and removed it again just to make sure it was
in the correct position, ( it only fits one way in hope ) just to recap the
ignition side is good fuel pressure is good when i try to start it, but it just will
not start,if the distributor was in the wrong it would put the timing 180
degrees out but it only goes one way. im now at my witts end , grateful for all
help.  Robin Mills.

Answer
Robin,
 I'll make this brief.  Let me know if you need clarification on any of it.  
  Verify the injectors are actually firing; take one off and crank the engine.
 Check the Cold Start valve in the same way.
 The best way to check the timing is with an inductive timing light.
 Make sure the fuel pump works when the ignition is on while pushing the air box flapper open with a screw driver.
 
Start with these and let me know what you find.

Rick