Sacha.
Have you come across these?
http://www.leevalley.com/garden/page...280,54307&ap=2

I've been using them on the pot plants in my kitchen window for several
weeks and they work! I have one for the rosemary and 2 each for the Bay and
Grapefruit. The latter is very thirsty, it seems, needing a daily top-up. .
When I leave for hols, I'll replace the glass jars that I'm using with a
couple of plastic buckets so that my elderly neighbour won't have to check
on them very often.
One is supposed to be able to use fertiliser solution as well but I find I
get algal growth in the jars in no time so I apply that directly.
Graham