Around the same time, Webbs introduced their "New Mammoth Red Tripoli" onion specially for autumn sowing.   This has since disappeared, replaced by improved varieties.   As it happens both Dobies and Suttons are introducing new red onions for 1993 Dobies have"Red Delicious" from sets and Suttons have "North Holland Blood Red -- Redmate".   "Redmate" is a dual purpose,spring-sown onion which can be picked early for salads or thinned to 3&ins (8cm) for mild, red bulbs.   But this is just one development following on from the introduction of sets, autumn-planted sets and, of course, F1 hybrid onions.   All are quicker to mature, bolt less and are more uniform in shape than old varieties.     mind you, with a hundred years of refinement and improvement behind it, you still can't afford to ignore good old "Sutton Globe".     FIRM FAVOURITES Globe artichokes are very unpredictable from seed, so Dobies have introduced young plants of "Camus de Bretagne", one of the finest varieties available.   I also like the look of their new Cos lettuce, " Corsair", which keeps well in the ground, has excellent disease resistance and received an Award of Merit from the RHS.   Mr Fothergills' new lettuce is certainly worth trying.   "Bambi" is even smaller, crisper and tastier than "Little Gem".