Thanks Vinlander. I think I will only use the crab apples for jelly. Am I right in thinking that the date when everything flowers is the key consideration, so if Bramleys are in blossom in May then I want my other apple varieties to be doing the same.
Hi Galette,
Yes, the big issue is timing so do use the RHS groups, but on top of that a few of the most popular varieties accept pollen but don't make any (triploids) - beyond that there are so few genuinely incompatible pollen makers that you would be very unlucky to plant just two of those and nothing else.
Incidentally the right pollen groups guarantee the maximum overlap of flowers, but they are wider than that, so adjacent groups can still work - just less reliably. If you only want to grow 2 types for market, then choose from the same pollen group. If you want just one type of tree then adding one crab is the neatest solution as long as it is close enough to all of them.
If like me you want a range of ripening times then you may end up with enough different apples to remove the risks.
I think the main reason crab apples make good jelly is their pectin content. It's only a guess, but I suspect they all have lots, and I don't think pectin has a taste, so there's no reason a crab apple that tastes good raw would make an inferior jelly - though I know a lot of people enjoy extra tartness in a jelly.
For cider (and many cider varieties are closer to crabs than eaters) the best mix includes sharp, astringent/bitter and sweet types - mirroring the importance of the sweet/sharp/toasted/bitterness balance that makes beers and ales so interesting (and bog standard lagers so disappointing).
I have a vague recollection that some cider varieties also have a usefully longer flowering period than eaters and cookers - but I can't pin down the source.
Cheers.
PS. I grow hardly any earlies because the flavour changes so rapidly at that time of year that they are disappointing unless you can eat them all off the tree or very soon after. I like a sharp crisp apple with a well developed flavour so my trees range from lateish to very late. On the other hand I eat them ASAP while that flavour is there - I almost totally ignore the storage idea - I'd rather juice them at their peak than let them get softer and sweeter. Apples from NZ etc. in April are so much better than being disappointed by my own stored apples - with the possible exception of Sturmers.
BTW ex-pats in France grow Bramleys almost exclusively as a zingy eater. I try that here but they tend to fall off while they are still green.