asda fruit trees!

It must be worth a punt, since that is the same as a couple of bags of Braeburn apples. The apple trees are probably a safer bet than the pears or cherries. Asda may be different but you probably can't make any assumptions about the variety or mature height.

Variety not named?
Rootstock not named?

How do you know what fruit you'll get? Cookers? Eaters? Ciders? Crabs?

How can you make an estimate of how big the tree might get, or how it might cope in your good/bad soil or climate if you don't know the rootstock?
Very dwarf M27 (about 1-1.5m), dwarf M9 (1.5-2m) semi-dwarf M26 (2-3m) to the medium vigour MM106 and M116 (about 3-4m), the vigorous MM111 (4-5m) to the very vigorous M25 (about 5-6m).

Rootstock is probably Quince A (pear), MM106 (apple), St.Julien A (plum), Colt (cherry) but there's no guarantee. I also find QA, SJA and MM106 almost un-growable in my soil as they are not vigorous or rugged enough. Others with good soil priase those rootstocks as "perfect".

my mum in law bought us 3 of them yesterday for christmas, a cherry which it says is a Stella, a pear which is a conference and a Bramley apple tree... i presume thats the same in all the asda's.. i think thats quite good as they are about 4-5 ft tall... well they are a present so hopefully they will do ok ... i'd give it a go Darcy

It depends if you have a spare spot to put them where it doesn't matter if they turn out to be useless. If you only have a small area then I'd say use it well. The top price for a well-grown fruit tree is about £35, which is the cost of a good pub meal for two. A fruit tree can live for up to a hundred years so the purchase price doesn't really come in to it, compared to it being the best choice for your taste and your situation.
The problem with a bargain tree is that it can take many years of care and lost space to find out that its not as good as you could have put in. I have a lot of room but even then space/time is precious - this winter I'm removing two £5 trees that I wish I hadn't bothered with, a peach tree that barely survived for three years then died this summer, and a plum that turned out to be rootstock. I'll have to wait now for several years to get fruit from the replacement peach that's an interesting variety on a more promising rootstock.

Standing back now for all the people who've bought trees from Aldi / Lidl/ Asda to tell us they've been fantastic!

Well, here's the first comment.......

I've encountered plenty of people who bought "discount" trees. It was like a lucky dip. Some trees were just right for their needs. Some trees got very large very quickly and wouldn't fruit (rootstock too vigorous). Others would not grow and therefore couldn't fruit (rootstock too dwarf).
Some sold as "apples" turned out to be plums. Some "pears" turned out to be apples. Some "Bramley's" turned out to be small, red-fruited crabs.

Having said that, though, I have had a few mislabelled trees from various nurseries. I've also had poor, diseased stock from certain nurseries - especially those that have been growing the same old varieties and rootstocks in the same location for decades. Dont farmers have a crop rotation policy to prevent disease? It doesn't seem to happen with fruit tree nurseries, who often grow the same thing in the same place each year, allowing build up of disease. Some nurseries appear to have established in locations which only encouraged build-up of certain diseases due to local climate.

The "old books" often praised fruit trees grown in Kent, as the soil and climate were usually perfect for producing young trees of the strongest, healthiest and largest quality. The quality of stock from Keepers is outstanding compared to other nurseries.
Buy a tree from Keepers and buy the same variety/rootstock from somewhere else - especially the more vigorous rootstocks where the difference is easier to see. Then put them side-by-side and see for yourself the reason why the Keepers tree was worth a couple of quid more.

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Here's a link to Keepers Nursery, for anyone considering buying trees this winter:
Keepers Nursery UK | Fruit trees for sale | Buy Online | Mail order

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well my christmas present trees will be going in big pots at the end of my garden and they will be taking there chances, if they do ok hurrah, if not i'll replace as needed... i might be lucky