Big thanks everyone! I just ordered a 30m for a start and I'll see how it goes. I can always add another if necessary. Any recommendations on adapters? I will be traveling throughout the EU.
I bought a 50m reel of suitable 3-core from an electrical wholesalers and split it into two halves. Fitting the plugs/sockets is straightforward and the end products are much cheaper than buying ready made. We've never needed more than 50m but there have been times when 25m wasn't long enough. We also carry a plug-in device to check the polarity and a short length of cable to reverse the polarity.
Like many, I've got a 10m & 25m. Plus extra "tails" (EU, reverse polarity, etc).