Access to exchange rates via web

I would like a worksheet function something like this: CurrencyRate(FromCurr, ToCurr, Date). This would have as parameters FromCurr (the three letter standard abbreviation (USD, GBP, EUR, DKK etc.) for the currency to be converted from), ToCurr (the three letter abbreviation for the currency to be converted to), and Date (the date on which you want the conversion rate). It would return a float which is the value you have to multiply by to convert the old currency to the new. The function should go off to some publicly accessible web service to fetch the rate, thus avoiding the need to download a massive table and store it in a workbook, and to frequently download updates to this table.

Has anyone done this?

Which web service will do this most effectively?

How do I write the VBA code to access the web service?

I would use this (for example) when I'm reporting on my expenses to convert them to GBP at the correct rate for the day on which they were incurred. I am supposed to do this, but to be honest it's too much work for me ever to have got around to it...

Thanks - Rowan