Thank you for the varnish info Lance. I just had a bit of a disaster with a Spitfire model, where I gave it a final coat of Tamiya Flat Base thinking it was a varnish, but as it dried and a whitish haze appeared I realized it was more of a primer!!!
Fortunately a clear coat restored things mostly and the slight haze gives quite a good scale effect - but I want to be in charge rather than trust to luck.
I'd picked up a bottle of the gloss polyurethane varnish but was a bit scared to thin it as I know how sensitive Vallejo paints are.
So, again, many thanks for the excellent answer.
Sandy