Thanks people, good leads.
Now I'm only left with the question what tank to use (small, medium or large). One thing I have to keep in mind is the lack of space in the bow, exactly the place I would like to place the tank (against the front of the WTC, just above the central retaining screw). I've experimented with a little spraying can of 250ml here, and it fits without problem. I've read I need about 500-750ml of ballast to get the U submerged, so calculating with 250ml of compressed air at 100PSI this would leave me with 1.9l of uncompressed air. This would give me 3-4 complete ballasttank-clearings before having to refill.
Anyone having experience with the size of compressed air tanks and where to place'm?
Bailed out aircrew please report here......