Ah, Rory has this problem in the CS8 but not so bad; the CDSW tanks had gone from tar to crystalline green stuff.

A large quantity of acetone works well, dissolves almost anything, but you have to be convincing with the local chemist to get a couple of gallons.

I may have not remembered this correctly but I'll see Rory in a few hours and re-confirm; our local garage man and classic restorer says a mixture of No.1 paint thinners and white spirit works very well.

No.1 thinners to us is the very ordinary stuff we use for all synthetic paint, red oxide etc and gun washing, it is not the stuff for acryllic or two-pack.

White spirit was also known as turpentine substitute some years ago, I have no idea what the rest of the world calls it. http://physchem.ox.ac.uk/MSDS/WH/white_spirits.html

The other trick to clean dried fuel tanks goes like this. Get/borrow/hire your domestic size, one barrow, electric cement mixer.

Find an old car or its windscreen (toughened) (possibly two) and shatter it into those tiny squares. Collect the glass bits.

Fit/strap the fuel tank to the mouth of the mixer with bungey cords or whatever to the interior mixing paddles, fill with broken windscreen glass, switch on and walk away for the rest of the day.

Remove, empty and blow out with the airline.

R.