Jon Parker wrote:

I've an opportunity to run mains power and CAT6 to a garden
summerhouse. Can they run in the same conduit? One cable each in
separate conduit? Get some shielded Cat6 and use that instead?

They can only run in the same conduit if both cables are mains voltage
rated, I've not seen any 230V cat5, but there is this

http://www.doncastercables.com/cables/17/77/EV-Ultra/Power-and-data-connectivity-combined-in-one-cable

Beware that one type has a single data pair (no use for ethernet) and
another type has four pairs.