Is there a boating equivalent of twin core and earth as used in domestic AC house wiring? Please point me to a website /source.
If so, I would assume the copper would be finer more flexible gauge to cope more with the vibrations and tinned for corrosion protection.

Or is it common just to use three separate marine class wires as used for DC in boats for the AC cabling for active neutral and earth?

domestic AC house wiring

Ive seen this used where I have my boat with a water proof out side plug in

Romans, tinning the end of a multistrand cable or any cable for that matter and then puting under a screw is illegal under the SA3000 wiring rules. Tinned 2.5mm twin and earth cable is available but very expensive and hard to come by. I would suggest using standard domestic 7 strand 2.5mm twin and earth (7/067) this is the cable most electricians use for power cable these days. Do not use 2.5mm single strand twin and earth (1/178) which is what is sold at Bunnings I think. Just remember doing your own 240 volt electrical work it highly illegal unles you are an electrician doing work on your own boat only or if you re a registered Electrical Contractor you can work on any one's boat. I employ about 40 electricians, I should know.
John

Thanks for all the ideas and advice.

I already have some domestic twin core and earth. It is 7 strand 2.5mm2 non tinned copper.
But I felt it was still a bit stiff and probably not ideal for use in applications where vibration and movement can be an issue.

I have some left over 4 mm2 tinned marine grade 3 core cable used for my auto bilge pump. You are dead right. Very expensive.
I agree, the insulation is for 12 Volt so again, not ideal.

From your responses above I think either could be used but I favour using the bilge pump wire which has many (4mm2) fine strands of annealed tinned copper.
The insulation looks as thick as the domestic AC wire. It might not be the technically perfect answer but I bet worse things have been used by experts.