I tested the sub in the bath last night and it worked perfectly. Took it to a display today and while it got some attention on the bench I really wanted to test it in the small pond we had. I ran into a lot of issues with the O ring, despite it having worked great yesterday. I cleaned it thoroughly to get rid of any grease, dust, water etc and it didn't actually work, it kept leaking when the hull was unpressurised to force air out of leaks. I then wetted the O ring with some water and by some miracle it sealed properly. Turns out the 212 is an utter pig to turn when it's on the surface with half of the x tail out of the water, but once it's down it turns much better and also becomes much, much faster when it is under water due to the hull shape.
She's also very bow heavy and needs the weight there reducing quite significantly.
It was a gloriously sunny day and the pool small, so the air inside the sub heated fast and tripped the pressure sensor within five minutes. I reset it successfully once, but it glitched the second time and I foolishly decided to turn the sub off and on again. It worked, but involved opening the seal which then refused to set.
It is one problem area on the lower third of the port side. Feeling around it on the outer edge of the area the O ring should sit there is a slight machining defect, but it shouldn't protrude into the area where the O ring sits when it's locked. I'm thinking of putting the thin film of Vaseline over the ring, since the water did seem to help it set.