Can I weigh without a set of scales?

This site will convert weights to volumes, and says that you should use .53 of a US cup.

So it turns out she doesn't have any volume measurements either, other than a measuring jug.

So in the end I divided the 500g bag between two identical glasses, then divided one of those glasses up between 2 others to get approx 125g

Now it seems that the cake tin doesn't seal properly anyway so I can't imagine that my cheescake is going to turn out any good anyway...

For Neil, a measuring jug:

Gourmet Sleuth can help you with its advanced conversion calculators.

For example, a teaspoon of baking powder does not weigh the same as a teaspoon of peanut butter. This calculator will simply give you an approximate measurement. When accuracy is imperative you should use the highly-accurate ingredient based calculator.

Weight To Volume Conversion Calculator shows that 120 gr sugar (powdered) = 1.2 cup or 48 teaspoons.

Just enter "sugar" as keyword and select "sugar, powdered" under "sweets" menu. Then enter the quantity, select conversion unit and click "Convert". On the left panel, you can see the results.

You can also use Wolfram Alpha for this, for example 120 gram/density of sugar/cups.

Although, they don't have castor sugar in their system.