Piriton knocks you the hell out, if you're not used to taking stuff. If you feel like going nature-boy, pick some nettle leaves and steep them - that's where you pour boiling water on them, not boil the leaves themselves. Works wonders, 5 or 6 good-size leaves in a mug will see you through for a good 6 hours or so. There's a wasteland near my place, and even though it's a bit of a hassle I find it totally worth the trip, especially as a supplement to the herbal stuff that's much cheaper than the name brands, which for the most part are all exactly the same stuff - cetirizine dihydrochloride (sp?) - and pretty useless.
For the record, I think Piriton is a different compound, which makes them more effective, but cause drowsiness.