Just finished the book (had my parents staying so it took a little longer than expected!). I'd read it before, quite a few years ago, but couldn't remember that much of it.
I think it was at its best with the descriptions of life in the trenches the sections featuring his granddaughter didn't really add anything. In fact quite a bit of the love story I found a bit soapy I remember now that his 'middlebrow' writing style started to annoy me after reading a few of his books. It reads like someone who has been 'taught' creative writing (as he was, at UEA I believe) -- a motif here, a descriptive scene there, don't forget to make some heavy-handed comparisons between the soldiers stuck in the trenches and the granddaughter stuck in the Central line.
Anyway, all that makes me sound like I enjoyed it less than I did. I thought the WW1 stuff worked very well and the end scene in the trenches was very poignant.