Td4 Misfire

2004 Td4 Automatic 110,000 miles

Yesterday my Freelander started misfiring on tickover, regular misfire, running on 3(?) cylinders. As soon as I apply any throttle it picks up onto 4 and appears to run fine. There is still enough power on tickover for it to start moving as soon as I put it into Drive.

A bit of background. Yesterday morning I made a 200 mile highspeed motorway run, no problems. Set off for home in the afternoon, all fine (well, I thought it felt a bit "flat" but I think that had more to do with me having spent the day driving high perfomance cars, I think it was going OK.) After about half an hour I came to a halt in traffic, and became aware of the vibration in the car. Opening the window and listening carefully suggested 3 cylinders, but as I said, as soon as I touched the pedel it smoothed out.
The drive to work this morning confirmed all is going well once it was under power. I did use a bit more fuel than usual yesterday, haven't claculated the figure yet, but given the nature of the journey, I would expect to.

A search has given me a couple of things to think about, but it is the fact that it misfires at idle, but works fine under load that has got me mystified.

Immediate thought is a leaking or faulty injector but then I would of thought it would also be difficult to start.
Identify which cylinder is misfiring by pulling injector connectors in turn.
Do a leak back test, if that shows nothing then I would try swapping the injector from the misfiring cylinder to another cylinder to see if the misfire moves to that location. Obviously that would confirm or disprove the faulty injector theory.