The rubber does tend to become very stiff in freezing conditions on the original type. The aero blades from Bosch seem to do a much better job, perhaps they are made from a different material?
http://www.ebay.co.uk/itm/Rover-MG-ZT-0 ... 257940d407
These are both the same length 550mm but should be fine as the originals are 21" and 22"