I bet the tweeter is blown. Unfortunately the tweeter is no longer available. However there is a
replacement tweeter available on eBay.
I had one of the tweeters on one of my SEAS dual concentrics blow. That unit is very similar to that KEF driver. The SEAS concentric is produced under license from KEF. I had no instructions, but I managed to figure out how to do the replacement.
Since the parts are NLA, the speaker exchange no longer offers repair of that driver.
If you order the part and don't feel you are capable of doing the job, I would be prepared to do it for you. I hope is is no worse than doing the SEAS!