Dave you are correct. Powerbox are a class act and in no way was this thread aimed at them. I still have confidence in their products and they handled my situation very professionally.
I sent the gyro back to Powerbox and they found the corrosion. They couldn't offer a cause or a prevention. All they could say was it wasn't from fuel as there was no oily residue. My situation was very bad luck and a one off so I don't think anyone needs to worry about Powerbox products.
My basement is completely dry and I'm very fussy about how I look after my planes. However I do live 400 metres from the beach. Could this have caused corrosion in a sealed unit? Maybe.
I just thought I'd ask the original question regardless of brands as this episode got me thinking, and worried.