You don't list where in the world you are located on your profile.
From the use of the term "nacker" I assume the united kingdom??
You aren't going to find a lot of options, 6 volt is old and limited in production anymore.
Here in the states we have two major brands and dozens of lesser brands!
Interstate &
Exide
https://www.exide.com/ Anyway you need a 19L battery.
All that are on the market over here have these ratings....
CCA @ 0 degrees 440 amps
CA @ 32 degrees (your zero) 560 amps
RC recharge time 120 minutes
Good Luck on finding a heavier duty battery though I don't know why you need one, what accessories are you running on a classic bug??
England doesn't typically have winters like we have here with extended periods of bitter cold and below freezing temperatures and even here, we don't generally drive these cars that have virtually no heater!
VW's redefined the term "driving gloves" when winter rolls around!
Dave
"If you don't have the time to do it right the first time, when will you find the time to do it over again later?"