As a result I've decided to base my British and Prussians on much thicker card - and place another layer of thin card on those already based.
So I popped into Liverpool with my eldest son to finish the Christmas shopping and got some offcut card from an artist shop I occasionally visit. This is now cut to size and I'll start using it tomorrow.
Meanwhile on with the current job.
Looks a bit sci-fi!
The picture above shows the Dutch base (of thin card) having another layer of card glued to it. The clips are from B&Q (only £1) and are great for this kind of job. Once dry the warping had gone and the base looked a bit more substantial.
The thicker based Dutch next to the thinner based Portuguese
I just need to finish drybrushing the current bases and flock them and they're ready for use. Having gone through the various trials, the rest should now follow a lot quicker.