Philip J. Ehlers was a postgraduate student (1989-1993) at Bangor, working in the team of Ronnie Brown and Tim Porter, who was Phil’s doctoral supervisor.
Phil Ehlers, Tim Porter, Varieties of simplicial groupoids, I, Crossed complexes, Jour. Pure Applied Algebra, 120 (1997) 221 - 233; Erratum: Jour. Pure Applied Algebra, 134 (1999) 221-233.