I personally use the place mentioned in popo's reply. Very good company to deal with and the process is very fast and straight forward.
If you plan to rent the lens from the beginning of the week they will post out to you on the Thursday before so that you receive it on the Friday morning to test, giving you a whole weekend extra in my books.
They have a great process where the lens is sent in a box with protective filters, return packaging and slips with the option of them collecting from your house.
Highly recommend company, i would suggest forward planning just to make sure the lens will be available for rent.
Lens pimp will do both camera and lens
http://www.lenspimp.com/