Humidity will do the work for you. Yours is not spared either.

Try Pasar Road. Some shops there sell the surrounds in various sizes / diameters.

Another home DIY fix would be to apply some Selleys flex-joint seal or whacchamacallit (i forget the exact name) but it's a rubber based flexible sealant compound that is flexible after curing.

I have experience the same problem. I purchased 6 pairs Wharfedale Diamond 9.1 since 2006 until 2010. The last unit I bought in 2010 is a bit different in colour and has no magnetic shield around the driver. This last unit got tear like yours.

I bought new rubber ring from ebay, and cut the whole things to replace with new one. Here is the link: -->Ebay (Rubber ring fits WD9.1)

Use the glue (white colour) sold at stationery shop.

Added :
When you get the new ring, and want to cut the old ring, do not attempt to tear off the old ring from kevlar cone. It will damage the cone. It wont work. Just cut the suspension area, leaving the remaining ring intact with kevlar. Then just glue the new ring on top of the remaining one. (What language  )