Pollination groups

Some of the more popular varieties are self fertile

eg Czar, Victoria, Stanley and if you like gages there's Oullin's GoldenGage

Pollination groups are to do with when the trees flower. In order to pollinate each other trees need to flower at the same time. Depending on he supplier classification may vary slightly but normally trees in the same or an adjacent group are OK. E.g. a tree in group B would be Ok with another in group A, B, or C but not D.

Some trees are self-fertile, which means they will pollinate themselves and don't need pollen from another tree. Yoj can plant a single tree if it is self-fertile.

The websites of many suppliers will tell you if varieties fhey sell are self-fertile, and which other varieties will pollinate them. For example, the Keepers website is pretty good.

Thanks Thelma and Chris, been reading up on Jubilee plum

Woo hoo! Jubilee is self fertile

If you needed a pollination partner, it must be a different variety. However, many plums are self-fertile or partly self-fertile.
If someone nearby has a plum tree it may act as a pollinator for yours, depending on where the bees fly. Sometimes there are wild plums in hedges, or plum-relatives which might pollinate your tree.

Plums are notoriously biennial, with a branch-breaking crop one year and nothing the next. That can foil pollination if your trees get out of synch.

The small numbers of flowers on young trees often mean pollinating insects don't visit them - if you were a bee, why bother to gather pollen from a few flower clusters when there's a huge tree down the road with more pollen than any bee could wish for?

Self-fertility in fruit trees is not a 'yes' or 'no' answer. There are varying degrees of self-fertility and the fertility will vary with weather conditions (hot weather can cause flowers to fade quickly and miss being pollinated, while cold weather can prevent pollen grains from fertilising flowers that they land on).
Even such factors as the weather and nutrients the previous year can affect flower quality in the current year, as can the rootstock onto which the tree is grafted and the rootstock's ability to search the soil to absorb everything the tree needs.

Thanks FB, should I choose a tree or cordon?

I would choose a tree rather than a cordon because you can mostly leave a tree to get on with it whereas the smaller cordon may need feeding, watering, weeding and pruning.
For a tree, you'll either need St.Julien A or Brompton rootstock, depending on your soil quality and the soil ability to retain adequate moisture. St.Julien is widely available.
Plums, especially on St.Julien A, are very fussy about having enough water and will not grow or crop well if your soil dries quickly and/or repeatedly in summer. In particular, the fruit will stop growing if the soil dries, then after the next rainfall, the fruit tries to catch up and swells so quickly that it splits and rots.

Thank you FB, invaluable advice

Worth mentioning that plums have a very short shelf life (usually edible for 1-2 weeks after picking before they ferment inside or suffer brown rot), so it might be better to grow several smaller plum bushes of different types, seasons and uses, rather than one tree that produces so much fruit all at one that you can't use it.
Several smaller bushes of different varieties would also cover pollination for each other and would smooth out the biennial effect.

This year I planted a guinevere plum as all the suppliers said it kept better than average refrigerated. So far it hasn't done well though... growth was almost nonexistent and it was the first tree in the garden to drop its leaves. If it survives and ever produces fruit, I will let you know in a few years if it does keep well.

The pershore yellow, also on st julien, that I planted at the same time a few meters away has shot out vigorously in all directions, so I doubt that the conditions are the problem.

Some varieties prefer certain soils, climates or methods of growing (e.g. feeding/watering/training). Some are faster-growing than others.
My soil is dry and infertile, with many fruit trees not doing well unless fed and watered. However, certain varieties and rootstocks have far exceeded expectations without needing any care, and in some cases completely defied what 'the books' said they would do.

In general, I've found 'modern' varieties don't cope well with not-so-great conditions, and tend to be weaker-growing and more prone to produce blossom/fruit buds than grow. However, the profusion of blossom doesn't result in a useful crop because the tree is too weak to mature the fruit.
It's probably because modern fruit growers want trees that don't get too big, and which they can feed/water to get the exact amount of growth/fruit they need.

Most modern varieties have a lot of hype. After all; the breeding stations have to fund their existence by selling new varieties, whereas nobody gets any royalty payment from selling an old variety.

Old varieties seem to grow well and crop well without much attention - not least because they had to survive on their own when we didn't have many sprays, fertilisers and irrigation to help keep them alive.

I would always choose an old and not very common variety over a modern one.
In a semi-neglected backyard situation, the old varieties cope better than the modern varieties bred-for-professional-orchards.
In fact, this evening we enjoyed a very tasty Belle de Boskoop apple crumble.

Let's see how well that metric does with things I've planted fairly recently. Here's a subjective rank of plants from most to least succesful, along with whether the variety is old (say pre-1950) or new.

MOST SUCCESSFUL

Iranian Medlar- unknown, introduced into the UK recently from Iran but might be an old variety there
Yellow Pershore - old
Howgate Wonder - old
Fiesta - new
Beth - new
Concorde - new
Guinevere - new
Stella - new

LEAST SUCCESSFUL