Would be interested in knowing which other music you (or anyone else) would compare Holt's with?
Tinners, well he himself would undoubtedly acknowledge the influence of PMD, and later - and far more centrally - Birtwistle. His teacher, Anthony Gilbert, I know had a major impact, but not so much in terms of his stylistic influence on Holt. Birtwistle can be heard, I think, particularly in SH's work of the 80s and earlier 90s, but thereafter his work becomes far more individual and harder to pin to any direct or obvious influences.
rm, I have particularly enjoyed the Two Movements for string quartet (actually quite stunning), and the violin concerto, 'Witness to a Snow Miracle'. Check out those gems. The 'opera', Who Put Bella in the Wych Tree, I found disappointing dramatically, but contained some tremendous music nonetheless.