Hmm, since I don't have the book or your reader, this may be a shot in the dark, but..
If I were you, I will try the following.
(1) Remove DRM from the book (Again, I have no experience buying books from Waterstone since I am in US so you may need help from someone with experience with Waterstone store)
(2) Then use Calibre to convert epub to epub.
The second stage may sound strange but Calibre splits content into smaller files so it is readable in machine with limited memory AFAIK. If the original epub is not well designed for small memory device, this may cure the problem.
Anyway, good luck.
