I have the same manufacturers Stormforce cover. Now bear in mind, mine lives outside all year round, under the said cover...
I've been disappointed with mine. Very disappointed. It's not been as bad as the German made Car Pyjama junk I had before, but considering it was their top of the range version, it's degree of water resistance dwindled very rapidly. I can't leave the hood down underneath the cover for instance, as any rain would come through fairly quickly.
The UV stable material has degraded fast, in just over twelve months its started disintegrating at the seams, while the solid panels of material have become very delicate, they now rip and tear at the slightest provocation - the last time we had snow, the freezing chunks of ice were pulling bits off as they fell away.
I'll be going back to Hamilton Classic's covers again. (Tried to get one at Stoneleigh, but they were waiting for a new shipment of the heavy duty cover).
Kept outside, all covers do age, and degrade over time, but the Hamilton ones are the only ones Ive found to give me more than eighteen months decent performance. (My last HC one was over five years old when it started to need replacement)