Thursday 2 February 2012 2.53pm
I've never found anywhere in SE1 but have managed to pick up the odd bits for my acoustic (plectrums, a capo etc) from the music shop at the side of the Barbican.
It's small and isn't guitar focused but has most of the essentials.
No connection etc. other than being a customer happy not to have to go to Denmark street!
Hope this helps