Try swapping the working leads over to the cylinders that wont fire, if they start working you know its ignition related. If not swap the plugs over and try again! Next pull the plug off the 5th injector, if that helps check its not supplying constant 12v at the plug, the 5th injector is very rarely used on these hot days.
If that fails then you're left with fuel or compression being the issue. Get a compression test done, make sure thats all good.
If that checks out, then pull all 4 injectors out and sit them in four containers of equal size. Next jump the fuel pump relay, my fusebox FAQ will help you find it! You need a chunky length of wire between the live and pump trigger pins, generally they'ree the 2 big chunky pins on the relay.
Now make sure non of the injectors start seeping fuel, if they do they need replacing! Then pop off the metering head boot and lift up the metering head flap, check all 4 injecors spray out a nice even cone of fuel, then when you're done, check all 4 containers to make sure they have all sprayed out roughly the same volume of fuel. If any are wrong I'd just replace the 4 as a set if its a car you want to keep for a while!
Then refit them with new injector seals.
If all THAT checks out you're left with a metering head or wur problem, for this you really need to get a fuel pressure gauge on it to check its all good, but you could just get some known good items and fit them. MK1s are notorious for fuel supply problems caused by rust & water in the fuel thanks to the filler neck going rusty!
