Others have explained the type of story that it is so I shall limit myself to a few opinions.
Good points
1) As a story it succeeds in balancing the "feelgood factor" with the fact that it is set against the tragedy of WW2. A difficult balance to strike but it does it well (although see Other points 1) below).
2) Yes, it is a sequel although this is not obvious from the cover, the synopsis etc. Unusually, this works well. The plot of this novel the mysteries of the original novel run in parallel and so in some ways it is like having two stories for the price of one.
3) It is written at a pace that is very evocative of the era that it portrays.
4) It is written in a style that is easy to read.
Other points
1) The only part where my heart sank was where one of the key characters was (apparently) killed during operations but miraculously did not actually die at all. This was not entirely credible and for a short period made the novel unconvincing. It is worth persevering though because the next event restores credibility.
Overall, well, this was my first Margaret Dickinson novel and it will not be my last.