I stand corrected MartinR, you are correct with the reference; I was following something else that Aymer Valence had written and crossed it over by mistake.
Either way, even at the time of F. C. Elliston-Erwood's article on 'The Apse in Kentish Church Architecture' (Arch. Cant. Vol.43, 1931), the remains at St Bart's, Chatham was recognised as being of twelfth-century and not earlier. It seems that the main reasoning for the dating of the St Bart's apse by Greenwood is the zig-zag decoration of one of the window heads, which must be the one in the left of the two late eighteenth-century drawings of the St Bart's apse? If this is the decorated window, with Greenwood's suggestive date, this would lead Historic England to use this a cited dating criteria for the construction of the apse?
Unfortunately, the adjacent excavation failed to clarify the date of the apse further, which means that regional trends, and comparisons have to be relied upon. As I mentioned earlier, the early Norman church buildings were not big on constructing curving apsidal east ends at a parish church level, so opted for the square ends instead.