BLUEMOMENT

I have recently found that my ongoing electrical problems might be due to volts going missing somewhere in the spiders web of wiring.
Today I discovered that from the main battery switch to the switch panel is a single cable - but it doesn't look particularly thick. I didn't have anything to measure it with.
Please can anybody tell me roughly what thickness the cable from the battery switch(and the corresponding cable from the battery) to the switch panel should ideally be?
The only things I run through the switch panel are lights(domestic & Nav), VHF, instruments and autohelm.

You need a meter - doesn't need to be a good one. Your problem is most likely to be connections, even if they look good, and a meter lets you find where the problem exists.
As to your question - over short runs at normal temperatures a reasonable working figure would be 1mm**2 good for 10amps; 2mm**2 for 20amps. If in doubt stick another wire in parallel but doing so will disturb the connections and may fix the problem.

I agree with getting a meter. Every home (& boat) should have one! For all they cost, the time saved getting answers instead of guessing is very worthwhile.

Hi Pete, take a look at the brown wire on the positive battery lug in your car (or on the starter solenoid) - and expect to find one the same or thicker in the boat.
As for meters, the cheap ones are cheap! They are accurate enough for our purposes, but tend to break easily - usually display and range selector knob failure.
If you're not happy with meters, get one of those screwdrivers that light up as a voltage probe, and wire up a high-wattage car bulb to test current-carrying capability.
Have you done the touchy-feely bit yet? Bad joints and too-thin wire get warm while they are stealing volts.
Also be aware of discolouration and smell (even noise!) in extreme cases. Use all the senses, Luke.