A couple of thoughts to add:
The actual volume of the cups are different in different countries (ranging from 200ml to 250ml) and as others here have said the method you use to fill the cups (mainly with dry ingredients) will often change the amount (e.g. dip and sweep vs lightly spooning, vs, packing). With wet ingredients the difference is slightly less variable.
Personally I prefer to measure by weight too but as you have found many recipes don’t give this (Rose does of course though - yay Rose!!)
This article does give some explanation and some conversion tables: http://en.wikipedia.org/wiki/Cup_(unit)
This site gives an excellent conversion table of commonly used ingredients by weight so might be of use too:
Also… one more thing, not helpful for cup conversion but useful none the less:
Did you know that Google itself has a amazing feature which allows conversion in HEAPS of things:
Just type in the search box and Google does it for you:
Convert XX to YY
E.g. convert 350F to C
convert 7 oz to grams
convert 1.5 kg to oz
convert 5 pounds to kg
it works for many units of measurement not just for weights and temperatures.