Phil 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.
His MSc thesis was entitled: Simplicial Groupoids as Models for Homotopy Types. (Thanks are due to Stephen Gaito for preparing a scanned version with Phil’s permission.) Here are the details:
P. J. Ehlers, Simplicial Groupoids as Models for Homotopy Types, 1991, MSc thesis, University of Wales Bangor, available here,
His PhD thesis was:
P. J. Ehlers, Algebraic Homotopy in Simplicially Enriched Groupoids, 1993, University of Wales Bangor, available here.
He has published several papers based on his work (jointly with Tim Porter). These include:
P. J. Ehlers and Tim Porter, Joins for (Augmented) Simplicial Sets, Jour. Pure Applied Algebra, 145 (2000) 37-44 (arXiv) .
P. J. Ehlers and 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.
P. J. Ehlers and Tim Porter, Ordinal subdivision and special pasting in quasicategories, Advances in Math. 217 (2007), No 2. pp 489-518.