Any good fertile soil will do, but the most common mistake is to dig a deep narrow hole rather than a rather more shallow much wider one, say around 4ft diameter. Remember that most of the important feeding roots of a fruit tree extend OUTWARDS from the tree rather than down.
Mix well rotted compost into the soil with blood and bone as a slow nitrogen fertiliser before replacing in the hole.
A 5ft tree is quite sizeable to still be in a pot and is probably quite root bound. When you plant be sure to loosen up the roots which are probably going round and round inside the pot, as they need to grow outwards.
To plant now, unless we have a wet summer, will be quite a shock for such a big tree, and there is a danger it will dry out unless watered very regularly (a good soak every week or so). I'd say it would be better to keep it watered in its pot until leaf fall in the autumn and then plant it as if it was a bare root one. That way it will have the cool, wet winter in which to establish itself.

Thank you 'Bertiefox' for your advice, especially with regards to not planting too deep as I would most probably would have done.
Again, thank you for your reply.
Much appreciated!

Further reply to 'Bertiefox'.
It is now July 22nd and needless to say, I didn't have to bother watering my little plum tree. Mother nature did it for me, 'WITH A VENGENCE'!! However, glad to say the plum tree is doing fine. (Roll on summer!)