I have long thought that manufacturers look at parts in a committee and think "How much can we rip them off for that" (e.g. Powershift oil kit £220 as opposed to £90!!!)
Whilst unbranded parts fitted in line before existing filter can't do any harm and may well get rid of any serious contamination, once that is resolved, any reasonable make of filter Mann, QH, etc should be OK.
I use Comline (I know that may be anathema to some) or whatever my local factor supplies and never had any bother - regular changes (2 years for fuel) are probably better that OEM and not changed - but that's just me!

Ok, I've finally got some spare time, so I'm going to attempt to remove the filler hose off the tank and see if I can get a pipe in to remove the fuel.

Do I need to pull the fuse for the fuel pump out and run the engine until it stops before I remove the filler neck?

If so, does anyone know what fuse number it is on a 57 reg MK2 tdci?

I'm almost tempted to skip this and just add the biocide stuff to a full tank of fuel, as I've run the tank as low as I dare, but I suppose I had better try and drain the tank... if I can get to the filler neck that is.

These don't use an electric tank pump so no fuse to pull.  You will need to manually prime with a hand pump after putting fresh fuel in.