Thanks.Dan, National Bike Tyres mail order, Speedmaster 20 inch front, Roadrunner 4.10 x 19 rear, £74 each, in stock. Rim tapes and Michelin tubes also available, ask about front tube size, current advise seems to be stretch 19". Very helpful, reasonable carriage costs, recommended. Good luck....
Yes, on Velocette owners site, consensus seems to be 19” tube for a 20” wheel. It may crease using 21”.
Wheelhousetyres say that the Michelin 21” tube is a perfect fit for the Speedmaster