Undoubtedly damage from the harsh winter we've just had just showing itself. The second stem looks fairly healthy and I'm not sure if what I see in the picture is extra growth at the base, or whether that's a different plant altogether. I don't know how dry the weather is up there - in London, another cause of this might currently be drought, it being like the Sahara down here.

Thanks for the response - yes, the winter was long, i am just rather surprised it has taken till now for the damage to manifest. The second stem is indeed of the same plant and is looking ok at present, though browning at leaf tips which i assume is lack of water. I just hope it does not go the same way as the main stem...