Dominic, Nick and Callum,
Some interesting ideas.
I’ve decided to opt for a middle of the road powered USB hub. If it doesn’t work then I’ve only spent £25. The computer is in the observatory at the moment, but I think 5m cables is just over the limit for stable usb camera connections. I’ll give some of the other suggestions a go if this doesn’t work.
I actually had 2 cat5 cables laid from the observatory to the house when I had the original observatory built. That gives me internet in the observatory, and also gives me the option to remotely control everything in future if I want to. However, they only go to the wall of the observatory, not to the pier.