Long story short, on Moat mornings this week it's been a tad colder (6 and under) the battery voltage first thing reads as 11.2 (marginally low) but on the cold mornings it drops to about 7 on startup resulting in a hard kick for my car to hit life, warmer mornings it starts by dipping to 9 volts. I have a spare bosch battery at home so will probably charge it and swap it as a precaution, but then after a 7 volt start, it doesn't have any low voltage battery codes anywhere...

Anyone know what the threshold is for this?

You'll always get the voltage reading dropping when you put a big load across the battery such as a starter motor. That's one of the biggest electrical loads you can subject the battery to. Plus as it gets colder the oil will be thicker so there's more resistance there too.

It's the steady state voltage off-load which gives better indication of the battery's health - unless the car is fitted with a battery monitoring system I doubt it's able to log battery voltage codes. If the steady state voltage is below 12.5V then there's potential for damage due to plate sulphation.

Steady state is bit tricky to measure once the engine has been run as this artificially raises battery voltage, but if it's at 11.2V either way it's likely past it's best. If you've already got a spare it'd be good idea to change it now before you get caught out once the weather gets worse.

Is there a way of "helping" a battery in winter?

Wrapping it in paper overnight? In a plastic box? a top and sides pvc covering?

Does that help?