I've just done this, so:
If car has recently been running, watch out for pressurised system. I forgot and squirted petrol everywhere as I pulled it off.There is a way of depressuring safely, can't recall how.
Make sure you prise out the old O-ring from the injector rail. It may not necessarily stay on the regulator pipe.
Check the manifold nipple is clear, and the rubber pipe a tight fit.
Good luck