Change your relays, at £10 each you now know (with the coil) that three of the four usual suspects are good. In my experience most Mk3 faults like this are related to the above or with a faulty crank sensor.
If they are all good then you can then go through injectors one at a time with a good unit etc, knowing your foundations are good is key in fault finding.
If it turns out to be a loose wire on the combobulator, then you have coil and relays for testing at a later date.