Best time of day for getting good exchange rate?

Howdy, 

Is there any sort of pattern for getting the best GBP to USD exchange rate when it comes to time of day or day of week?

I've got one last US bill to pay via wire transfer - and I don't know anything about finance other than basic personal money management.

thanks

I know this is of little use to you now but if you have a current (checking !) account with www.nationwide.co.uk and their debit card and you use the debit card to pay for something then you get the best spot price exchange rate at the time of the transaction i.e there are no exchange rate charges or back loadings. HTH