All the camera retailers seem to be doing this lens with 5% discount code at the moment but, when I was about to buy at Calumet, a helpbox popped up on my screen and informed me there was 10% off today with code CANON-L-10 which brought the £675 lens down to £607.50, the lowest price ever according to CameraPriceBuster.co.uk (Jessops sold at this price back in Oct).
The pop-up message said "Did you know you can save 10% on all Canon L series lenses? Use voucher code CANON-L-10." So should work for all L series.
3.8% cashback available at Quidco; and
Canon winter cashback offering £160 on this lens.
Hope this is helpful to someone.