Trevor started this method last year as a good way to water his Alpine sinks. His are planted with a mound of soil and dressing stones well over the level of the top of the sink which makes watering difficult as it tends to run off before it gets a chance to soak in, but does look good, as those of you who have seen his talk on 'Sinks and Troughs' may remember
Trevor took 4 pint plastic milk bottles and pierced a small hole in the bottom. These were then filled with water, the tops screwed on and balanced, often rather precariously on top of preferably stones or rocks in the sinks but sometimes on the actual plants and then left to drip through slowly over an hour or more. Not a drop ran away as it soaked in slowly drop by drop. Mind you your feet can get quite wet as you carry them to wherever you want to position them. I've got round that by carrying a few at a time upside down in a bucket so you don't lose any water before you get there. You may need to loosen the cap a little if the water doesn't run through.
I then found some special spiked tops with two holes at the end of the 6" spike which fit onto either 1 litre plastic bottles straight away or using an extra screw top would fit a larger bottle. These are easier as you carry them the right way up and you don't lose any water until you push the spike into the pot, trough or sink you want to water, and even better they can be pushed down to the roots of special plants to give them a whole litre of water around the root and not a drop wasted. Unfortunately I can't remember where I bought mine, possibly at Workbridge, but they have some at 'Neat Ideas' Water Spikes 6 pack for £6.99 and of course they will last for many years once bought.
Both very successful. Look for pictures in Members' Photos under Watering.