Calculating Euro values for de-minimis declarations

According to the Department for Business Innovation and Skills, when you are calculating the value of de-minimis aid you should do so using the exchange rate that was current on the date of the written offer:

What exchange rate should I use?
The official Commission exchange rate applicable on the date the offer of de minimis funding is made (date of written offer) should be used. The rate is updated monthly and can be accessed here.