several fruit trees

Not a complete answer but you could plant hazel as your hedging and put the fruit trees in front as your top layer. What about the rootstocks of your trees?

preferably a rootstock that means they won't take over the world..Something that will keep them to a max of say 20 foot or slightly smaller.
I've read you can get vigorous trees on dwarf rootstocks that will be the same thing as putting a normal vigorous tree in less than friendly soil...but the soil is clay and very tree friendly judging from the success of the elderberry and other trees in that square.

I think hazelnuts/cobnuts are good with fruit trees as I've seen them together elsewhere. I'd really love a walnut but they are not good to others around them, so I have to make do with the one up the lane. I have planted apples, pears, plums, quince and medlar in mine. Still looking for a mulberry that's mature enough but doesn't cost the price of a kidney. Nothing seems to hate one another yet.
My grandmother had an almond tree in her garden, but they get peach leaf curl.

I tried an almond, it didn't make any effortat all. When I was discussing it at the nursery they said that a couple of hazelnut trees would be better. So that will be this years experiment.

Walnuts and conifer-types are not good companions for planting as their roots and fallen leaves contain substances intended to poison nearby plants, or at least significantly slow the growth of nearby plants to reduce the competition.

But apart from those, there isn't usually a problem with companions.

I would suggest alpine strawberries as ground cover, with currants and autumn-fruiting raspberries as shrubbery (autumn types are self-supporting if sheltered; no need for post-and-wire), then the fruit trees growing above (half-standards or standards with 3-6ft trunks).

Use hazels as hedging/windbreaks. Hazels will grow almost anywhere - they're woodland edge bushes which tolerate very infertile soil, competition and partial shade. If the soil around hazels is too good, they will be slow to crop. "Wild" hazels will take a long time to start cropping and will have nuts of unknown quality/quantity, but commercial varieties such as Gunslebert, Butler, Gustav-Zeller and Kent will crop earlier and heavier.

If the soil is good, the common "medium vigour" (about 3-4m in size in average soil in 10-15 years) rootstocks such as St.Julien A (plum), MM106 (apple) and Quince A (pear) should suffice. However, you seem to be having problems with a plum, although rootstock is unknown whether Pixy or St.Julien.
If a plum on St.Julien is struggling, you may not has such good soil as you think.
In certain areas, the soil can be very poor (or an extreme pH) but the local plants (weeds) may grow well because they're well-adapted to the conditions.
So find out what rootstock your plum is on, and/or do some soil tests for pH and nutrient content. I fear that the poor plum growth signals that something isn't right.
For example: are you sure the soil is clay and not a grey chalk (with high pH which is toxic/reduces vigour to many fruit trees)?

If the soil is not as good as hoped, I would suggest looking at a stronger rootstock.

We have hazel and plum in our hedge-to-be at the front - all are growing well as they do in the wild. Much too small to have fruit yet though!

In front of this embryo hedge (i.e. nearer the house) we have planted wild strawberries (which are taking over!), gooseberries, and blackcurrents, all of which appear to be fruiting well. (Kinda what FB said).

I would love a Mulberry, but bear in mind the fruit, while delicious, creates a bluddy awful mess on the ground when it drops off, so you may not want this too near to areas you want to sit in or garden under.....

Mulberries - wot PnK says ^^^^. A friend has one in the front garden, overhanging the drive. Its a no parking zone when its fruit time.

I would be underneath with my mouth open and skyward. I love mulberries.

errr...thanks FB, I was hping you'd chip in
The plum is struggling for several reasons. The first it was badly kept in a container for about 3 years, then planted out. What I didn't know at the time and now do is that there was a concrete path to the side of it, about 6 inches down which has inhibited it on one side because it's planted perilously close to it, if I'd moved another few inches, I'd have noticed it straightaway. The other is that it fruited really heavily the year before last, when it had only been in the ground for a year, and it puled it out of shape, the fruit weighed all the branches down and it hasn't recovered yet. It's sent out a few brave new branches that I pruned last year, and it's looking a lot more even this year. I'm going to rub off as many fruits as I can this year, to see how it does next year. And to give it a chance to maybe stabilise a bit. The concrete path is going too. It'll make some bed edging somewhere else.
I probably will still do a soil test, there may be more paths lying about I don't know about
And thank you for your as usual sage advice
I've started a new gardening book so I'll be scribbling it all down in there tomorrow

Couple of thoughts that may or may not help - concrete, as well as stopping rain getting into the soil could be leaching lime into the soil which could affect the pH round it.

It does sound like your tree may have over fruited and that it's now putting it's energy into making roots after being kept in a pot. The fruit bending the branches down (festooning) woud not be a bad thing as it tends to encourage the branches to fruit instead of growing upwards and outwards but if it's a young tree then it should probably be left to not fruit and grow to the desired size before this happens. Having said that Victoria's are prone to this as they fruit so heavily.

I had a Victoria plum that trained itself into a weeping form but I left that to it as it kept it nice and compact and produced enough fruit for me. Once it started fruiting properly, it fruited well like this every year without me doing much to it (although I did get drought stress on the fruit as it was in the rain shadow of an oak which sucks all the water out of the ground once the leaves are out, plums do seem to need a lot of water)

Oh yeah and hazel nuts are all well and good being easy to grow but if you've got squirrels then you'll probably never see any nuts! Think I've had a couple in the few years I've been growing mine. They are good for wood and windbreaks though

You'd soon be a lovely puce colour then! And very attractive to wasps....

I am anyway