I’m undertaking a bit of rewriting and retrofitting of various parts such as trailer auxiliary socket and some lighting. I am on the lookout for some plug and socket connectors rated to at least 30A per circuit (rather than the 12A the eBay specials have!). Anyone have any pointers please? Many thanks.

Anderson plugs are rated at 50A usually. You should be able to find them on the net easily. If not, sing out and I’ll grab some here for you, they grow on trees about this country.

If you don't like the Anderson option then these might be worth a look:
https://www.te.com/usa-en/products/connectors/automotive-connectors/intersection/rugged-circular-din-connectors.html?tab=pgp-story

I've decided to go for these instead of the usual Anderson connectors for my dual battery installation.
Ask if you want some clarity on these, the TE website & catalogues are a bit confusing if you aren't used to working with them.