Update:
Took her to the pond today, all works well, batteries have been replqaced with 12v nimhs in the bow similar to my Trafalgar, diving / surfacing time less than 10 seconds with plenty of ballast in reserve. unfortunately, bad weather meant limited time on the water & no pic's, but I will post some as soon as I get the chance.
I've tried Pistons, compressors, & Gas, this ballast system is, in my humble opinion the simplest to build & the most reliable, now to put one into my Gato.