I've heard this from Neil Huntley of Hartside Nursery:
"I noticed a post on the forum regarding Dwarf Rhododendrons and noticed a mention of Alan Clark and realised that maybe people were not aware that Alan passed away last autumn."
Screen shot below of the announcement.
Neil also says:
"I gather the young chap who was working for him may be carrying on selling the stock, but have no other news - we may see him at an event in the next few weeks - but not sure how he will be able to continue with the propagating etc without Alan around."
Thanks to Neil for passing on this information about the passing of Alan Clark.