Well, it's after 2:30am and I should be headed to bed ... but a few brief thoughts.
Well, the theme suggested sci-fi and that's what we got (close enough).
I liked the idea much better than the execution. The idea that your mind/awareness might occupy a different time to your body was, I thought, quite fascinating. The drug thing was a bit iffy with regard to an explanation for it, but du Maurier was missing some of the technical possibilities that have since been explored - thinking here of the end of
The Light of Other Days by Arthur C. Clarke and Stephen Baxter, where they follow genetic links into the past; or
The Trigger by Arthur C. Clarke, Michael Kube-McDowell, that ends up exploring the information theories behind DNA.
Speaking of genetic links. I was expecting an explanation for why both Magnus and Dick should be tied to the same person (Roger) in the past - perhaps that the three were all related. But such an explanation was never made explicit, and I wondered if we were supposed to assume it.
I liked Vita and the kids, but felt as if they were kept out of the central story more than was realistic - presumably to stop things getting overly complicated.
Dr Powell was a problem. I had expected to discover that he had a connection to Magnus, as that would have explained him much better than the ... what? Is he just a busy-body with convenient qualifications? Far too convenient as he was, whereas there seemed no reason why he should not have had some connection with Magnus.
I liked the ending. I didn't find it overly powerful as such, but it was satisfying.