Many thanks Jenny, I knew you had some good designs for multi-band verticals!
I've solved my problem and have just ordered on Tuesday this week a Spiderbeams 12m fishing pole so will simply run a 30m GP radiator down from the top and a 20m radiator offset by a few cm parallel to it. The radials will be attached about 4.5m up from the base and slope down at about 45 degrees. It's not a very light and portable antenna like yours, but will be great when go go the island of Barra in April. I've found a cottage with the ideal location for playing radio!
Have a great Christmas and I hope to work you again sometime in 2017.
73/72 Dave