Hi cockney, I have a tree fern in my garden that needs to have it's trunk kept constantly wet due the it's Australian rain forest origins.
One simple and effective method I have found which is also very cheap, is to fit a small plastic tap near the base of a square plastic milk bottle and attach a legnth of tubing to the spout that runs down to the plant.
The tap can be finely tuned to alow the flow you require ie 1 of drip of water per second (that should last most of the day ) and is powered by just air pressure and gravety, the cap can be left on the bottle to keep debris out but must be kept loose otherwise it wont work.
I bought the tap for about 50p several years ago at a garden show, but I dont know where you can get them now but somewhere that sells wine making equipment, aquatic store or garden centre might be good places to look or maybe see what Google can find. Good Luck. Tbird+