Hmmm... Harry Enfield plays caricatures, not characters. I don't think he's going to play someone that nuanced, and some of the humour is very subtle - stuff that starts as a vague suspicion or a sense of oddness, then dawns on you - not the style of comedy that Harry Enfield does at all.
Plus - or multiply and exponentiate - Dirk Gently is very, very bright. Not someone playing 'clever git' to the audience, but a man of arcane knowledge, lateral logic and dangerous insight who disarms the wariness that this can create with good-natured and offbeat humour.
An aside: Somehow, Dirk Gently always stepped out of the page as Ford Prefect (or rather, David Dixon's portrayal in the BBC television adaptation of HHGTTG). Who would play him now?