"Paul Luton" wrote ...
We have just finished our first early potatoes, (Rocket). There were not
very many per plant but very large - up to 1 lb each. Is this a
characteristic of the variety or something about my cultivation ?

My booklet says...

" Rocket.. (1987) Possibly the earliest early around, producing huge crops
of uniform attractive white tubers."

So you probably left them in too long before cropping.

Suggestions for next year....
"Belle de Fontenay" or it's offspring "BF15" (slightly bigger tubers).
"International Kidney" (Jersey Royals but needs seaweed manure to get the
true flavour)
"Duke of York" (famous for flavour, can be floury wel old, also a Red D of
Y)