Mk2 Fuel tank advice

You can get name brand tank cleaners but they are targeted at bikes not cars and are not cheap. I`ve found vinegar works best, you need about 20 litres. Get a dishwashing brush and fix it to some rod so you can get into the tank and scrub it. Put 10 litres in the tank and scrub the areas you can reach, leave the vinegar in there for a week giving it a scrub and a good shake every  day. After a week drain the vinegar out and if you have access to a presure washer use that to clean as much of the inside of the tank. Now put the other ten litres in and seal the tank up so it doesnt leak when turned upside down, leave it for another week (if you can get inside during the week give it a scrub).
 Drain the vinegar out and then fill the tank  with water , add about a quarter of a box of baking soda to neutralise the vinegar and leave for a day or so. Drain the tank and leave it in the sun to dry  properly.
 You should get most if not all of the muck out.

I recently pulled my tank out and found that the rattling was just bits of solder (from the manufacturing process presumably).  I expected to have to clean my tank out, and was considering using this:
 
http://www.kbs-coatings.com/tank-sealers.html
 
but after peering down the filler it looks clean enough as is.  Seeing how it was applied in a YouTube video also put me off a bit.  You could just buy the cleaning agent and skip the sealant  I guess (there's a rep for KBS in Malaga).
 
As for the rubber seal, they became obsolete a very long time ago (I know because I asked a very long time ago), although you can probably find something the same size if you look far and wide enough.  I've currently got mine stored in a plastic bag with a little 303 Aerospace Protectant sprayed in, just to try and keep it supple.