I thought Jon Doran's article "Beginners' Basics - Summer Packing" (http://www.outdoorsmagic.com/localiser/member.asp?SP=&v=1&memNo=16) was useful but maybe a bit 'academic'?
3 litres of water might be ideal, but it's also heavy (3kg, Sherlock . For your average 4 hour plod (beginers stuff), surely a litre of water in a squash bottle and a flask of coffee between two is more than realistic?
For an average day out, I'd suggest:
Wear a decent base layer, a light fleece and tracksuit bottoms
a lightweight waterproof (there's plenty for £50 from top brands)
A map/compass (if you can use one)
A space blanket
A litre of water (an old squash bottle is fine - just top it up in streams)
A flask of coffee
A sandwich and a few jaffa cakes
A handful of jelly babies, and maybe fruit/nuts
Lightweight gloves (liner ones are fine in summer)
A whistle
A couple of band aids
that's it really
the whole lot will only weigh 4 or 5 kilos