Wikos have new covers also you can get them on eBay.

Also Amazon Dave and I bet cheaper than buying a zip of that size...

Just a thought Dave. Be careful of the quality of your replacement cover. It may be more cost effective to buy a good quality one from Gardman for example rather than a cheap one from Wilko. I speak from experience as the cheap one split in the first strong wind whereas the new one has stood up to the wind so far.

I agree about buying the better quality one too..My 2nd one,is a reinforced one with mesh.not a clear see through one,but a darker green colour,which also helps to protect from hot sun..when we get some..! I don't remove it in summer,but just roll it up,and tie the attached ties around it..