Relax Tim - no need to stop the presses. From
http://www.thevulcancollection.co.uk/xh537skybolt.htm
One external indication of 537s Mk1 inheritance was its narrow air intakes. These were suitable for the mass-flow of the Olympus 201 engines but were considered too small for the 301 development and B2 aircraft from XH557 onwards had wider intakes anticipating the larger engines. A plan to enlarge the intakes and retrofit Olympus 301 engines was rejected on cost.
However, I seem to remember that 'some' of the new aircraft first delivered to Coningsby (1960'ish) were rotated through an engine upgrade 201's to 301's.
Brain's going here - but is it possible that, due to a shortage of 301s, some 301 capable aircraft were delivered with 201's as a stop gap?