Hi Donald,
What you describe is a very old tip which has been around for a long time, as you have discovered when soil in containers gets very dry it cracks in the top and shrinks around the sides, any water you then add just washes straight off or straight through.
Using a few drops of washing up liquid (the stuff for washing dishes by hand NOT the detergent that goes in a dishwasher) helps to emulsify the soil (is that the right word?) so that the water can soak in again.
I haven't tried it personally as I live in an extremely wet area but I will look around on the British websites for more info for you.
Edited to add--
Hi Donald,
Just had a quick look around. Here they recommend adding the washing up liquid to the water (a couple of drops per average size watering can -around one gallon in UK) and watering it in.
Another thing which you can try is pack in sheets of newspaper between the soil and the container wall when it has shrunk away so that when you water the container the paper gets soaked and stays wet and this gradually rewets the soil so that it can be worked again.
Hope this helps.
