I followed up on it with the council staff responsible for the park. It seems the dirt is here to stay for two reasons.
First, cleaning the statue carries a risk of damaging the memorial which, obviously, we want to avoid.
Second, the build-up of dirt is seen as symbolic of the dirt, mud and smoke of the trenches. While I’m no artist having had it pointed out I can see how it adds to the drama of the memorial.
The artist’s daughter lives locally and the council talks to her about the statue (she originally advised the council against cleaning). We are also looking at getting a consultant to advise on the long term maintenance of the parks statues and sculptures. So while the memorial might be getting the occasional brush-up it won’t be getting a deep-clean any time soon.