Thanks for the suggestions guys, since I originally posted i have tried wood sorrel, Scots pine needles and blackberry leaves.
The wood sorrel and Scots pine were made with boiled water from a thermos. The water was plenty hot enough but neither gave off much flavour. They were steeped long enough for the water to go from boiling temp to drinkable so maybe I didn't use enough leaves and needles?
The blackberry leaves I bought home and poured boiling water onto them and steeped. They provided a flavour which I can't really describe. I'm not sure if I liked it or not. Guess i'll just have to try again!