David in Normandy writes
Would I be correct in saying the key thing for pollination
of a bramley apple tree is that there is another (different
variety) apple tree in flower nearby at the same time?

Yes.
Except that Bramley is odd in that it won't itself pollinate another
tree, so is the tree that you are using to pollinate the Bramley is
yours, you'll need a third tree to pollinate the pollinator.

My books made apple tree pollination sound very complex
with lists of what will pollinate what and whether is is in
group one, two or three, but a web site I've just visited
implied that it simply comes down to having apple trees
that flower at the same time - is this the case?

Yes.

What the numbers are all about is telling you when the trees flower, so
that if the tree you are interested in is a 3, other 3s should be out at
the same time and will pollinate it, 2's will be a bit early and 4s a
bit late, but will overlap so will do at a pinch, and 1s and 5s probably
won't be of any use because their flowers won't be out at the same time.

The numbers mean you can buy your trees at any time of the year, instead
of having to wait till spring and buying two which are the same stage of
flowering ;-)
--
Kay