The following is from the 1989 book Great Rock Drummers of the Sixties by Bob Cianci.
(this section also includes Mick Fleetwood, Buddy Miles, Mickey Waller, Carl Palmer, John Bonham, Aynsley Dunbar, Levon Helm and others. It also unfortunately credits Bobby Harrison with playing on AWSoP - an error the author wishes to correct if he's able to publish an updated edition):
BJ Wilson is certainly one of the most underrated rock drummers on either side of the Atlantic. He joined Procol Harum shortly after their first album was completed [sic] and stayed with them until their demise in the Seventies. He previously worked with an R&B beat group called The Paramounts, whose singles are extremely collectible today.
With Procol Harum, BJ consistently provided appropriate accompaniment, from the classically-inflected Conquistador, to the hard rocker Whisky Train.
Hopefully, he's still musically active wherever he is. [Amen to that ... Ed]
BJ's page at BtP