I wrote:

I had severe microbial problems last year (there's a thread somewhere here about it).

The sludge grows in the tank, and once you have it you can only get rid of it by dropping the tank and manually cleaning it, flushing the fuel lines, and changing the filter. If you are infected it is likely that you'll need to fit a new Volume Control Valve to the fuel pump as well, since they are very easily damaged by moisture 9and the sludge indicates moisture). A faulty VCV will cause awful running and dirvability issues but may well not produce fault codes.

The cause of the sludge is a microbe which can breed under ideal conditions at a rate which means a single microbe could produce enough to completely fill your tank in 24 hours! Fortunately conditions in your tank aren't ideal, but unfortunately they're not bad either. The microbes feed on the oxygen in moisture in the tank or in the fuel, and form the black sludge. There are biocide additives you can put in the tank which kill them and break them down to a size where the microbe cadavers will pass through the syste, and be burned, but these are better at prevention than cure.

The best way to avoid the problem is to avoid cheap fuels or badly stored fuels, but decent diesel from outlets which shift a lot of fuel (fuel distributors tend to add fungicide anyway). Avoid fuel from cans like the plague if you can, especially if it has been stored for more than a couple of weeks (consider adding biocide if you have to use that old jerrycan of diesel).

Also, on a 2008 Defender check where the outlet of the tank breather is. The early Pumas had a brilliant masterstroke of design and routed the breather into the R/N/S wheel area, either just in front of or just behind the mud shield that protects the R/N/S lights. In other words it gets all the road muck thown onto it, and if behind the shield it lives in a world of wet filth! it is almost guaranteed that you'll get a microbial infection with this arrangement, and I wouldn't mind betting that your breather ipie is full of sludge in this case, certainly mine was.

If you reroute the breather (mine is now in the rear body by the light units), clean the tank, and are sensible with you fuel choice you will probably have no further trouble. Oh, the less time the fuel is in the tank, the better too, and the less air space in the tank the better.