Lichens that may be found growing in Churchyards &c. of City of London' Churches

We are working on a Project to hopefully identify some of the Lichens that may be found growing in Churchyards &c. of City of London' Churches. Including possibly on the Churches themselves? 

We seem to remember seeing some in the small churchyard of St Olave Hart Street: and will be revisiting to see what we can find. See below for the Seething Lane Entrance to the Churchyard:

It occurs to us that we might think about another Project viz. some Lichens of the Canary Wharf area! There have to be some in that Concrete Jungle? Don't there? But we can think about that after finalising the City' Churches Project.