After several days of testing in the bath I think I'm getting there! As Mankster hinted it rolled on its side on the first immersion and even when propped upright was also sitting far too high in the water. The solution was to gradually add pieces of lead sheet below the WTC and in the bow and stern until it sat vertical and a bit lower. There was a problem in finding an adaptor to connect the airbrush propellant can to the tyre valve on the gas tank but SubTech
http://www.rcmodelsubs.com/acatalog/sitemap.html came up trumps with a supply can adapter. Ordered it and received it in 48 hours!
Another challenge came on the next test when the thing proved quite happy to submerge - but wouldn't blow its tanks. Two days later and after much fiddling with the valve timing it's now working fine I'm glad to say. Submerges and pops back up quite happily. Now I'm about to fix the ballast in permanently and do some more tests to ensure 100% integrity before venturing to our pond with it. Pictures will follow.
As an interesting interlude I've also spent 2 mornings ballasting out a Sheerline Akula belonging to a friend. It works too now! Let's just hope the first sea trials of both subs aren't too eventful.
Andrew