My '94 SRD suddenly started using more fuel a couple of months ago, the MPG dropped from the usual 40/42 to 30. I've checked everything I can think of, there are no leaks that I can see and I've changed the fuel, oil and air filters and of course the oil. I also added a bottle of injector cleaner at the last fill up which I do every now and then anyway. Now it has started idling very roughly once the engine is warm and I mean rough, the whole car shakes. I put it in early for the MOT last week and it passed with just an advisory for a small leak from the exhaust. I thought it would fail as it was attempting to shake itself to bits. It passed the emissions test with flying colours and the MOT inspector said he couldn't fail it for being "a bit rough". He said it might be sucking in air through a small crack in the fuel lines so I sprayed loads of WD40 around them to see if the revs picked up but nothing happened. Any ideas as to what I could check next would be appreciated, thanks.