As blknight pointed out its not uncommon to get crap in the water trap near the fuel tank and I clean mine out every time I replace the fuel filter. Usually it has some weird looking clear jelly like stuff which I can only assume is some sort of algae that lives in diesel. I also fitted an adapter ring to the fuel filter housing so I can use the cheaper Lucas CAV filters. The added bonus is they have a clear glass bowl so you can see if there is fuel starvation or water etc getting past the water trap. Mines payed for itself already as I got 10 x Lucas CAV filters for under $5 ea on ebay and have used five already.
I got my filter adapter here but i'm sure you will be able to pick them up in the UK as well.
Berrima Diesel Service - Australias Premier 4WD Service Centre
Cheers
Mark

