I was going to ask, 'Why do you think it's Green-backed, not Grey-backed?'
Because: according to 'Birds of East Africa', 'Green-backed birds occur in southeast Kenya, south to eastern and southern Tanzania'; and Green-backed is generally treated as a subspecies of Grey-backed by most books and most of us.
But then I looked it up on Cornell's 'Birds of the World' (online only). To my great surprise, this gives the species as Green-backed Camaroptera, with Grey as a sub-species or alternative name (but a search on Grey-backed Camaroptera produces no results). !!! See attachments.
Maybe this is why Bentley asked the question this way.
Anyway if it is a Camaroptera, it will be in the grey group in that location.
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