BBC reported that some of the queens caught earlier were descendants form a nest eradicated last year .i.e. overwintered queens. I doubt they will be the only ones.
According to DEFRA
"For a species to be considered as established, there should be evidence of a reproducing population having been present in the wild for a significant number of generations and which is considered to be viable in the long term without any human intervention".
Are they having a a laugh? For them to say that Vespa velutina is not established in the south of the country is unreal.
As you point out Nigel the ones that have been caught are probably just a small proportion of those that over wintered sucessfully.