I have just made my custard and it didn't split (but it usually does, hence I know what to do).
Turn the custard into a fresh bowl and put the bowl in the sink filled with icecubes and cold water (sink, not bowl - oh you know what I mean). Then whisk with a balloon whisk and Delia promises all will be well. If not then a spoonful of cornflour added to some cream whisked in whilst re warming very gently.
My success rate with these methods is about 50:50.