Well, the WJ broke down again. This time it seems there was air getting into the fuel line. I need to replace the diesel fuel tube going from the top of the fuel filter to the low pressure pump. I've had some trouble getting hold of parts quickly until I spoke to my local Merc garage as they share the same parts albeit with different part numbers. To cut a long story short I've got two part numbers which the dealer reackons will fit depending on if it is an earlier design or a later one so I'm going down to a Merc garage which has both in stock to check them out. The part numbers according to the dealer are:
6120701432 - early models
6120703132 - 2002 onwards
When I find out which one fits mine I'll repost. Fingers crossed this works!