I'm also surprised that the late arrivals could not now be accommodated at SEN following the revision of the fire cover categories. I'm not sure who pays the cost of such diversions as I recall hearing that in some circumstances it is Stobart in the case of easyJet diversions. I heard that when the RVR is below CAT 1 but above CAT 2 may be one such situation where that applies but I cannot confirm if that is correct.
The Flybe LCY-DUB services on Good Friday using SEN instead seems very odd but little surprises me as far as Flybe are concerned these days.
Edit: LTNman - delayed scheduled flights are exempt from the night flight restrictions.