A5 (5.83 × 8.27), is not a "standard industry size," but if that is the size of your book, you enter that size, wherever it is that CS asks.  Then if that is the trim size of your book--if it matches your file's size--that's it, it works.  CS will print any size between 4 x 6 inches to 8.5 x 11.69 inches. (The biggest problem with uploads and book data, is entering the wrong trim size as compared to the book file.)

Actually it looks OK, except for too much space at both top and bottom  of page, which I've tried to solve by using a header, but it doesn't  look quite right.Is this something to do with trim size, of which I lnow  zilch.

What programs are you working in. What are you uploading? Where is it that the pages "dont' look quite right"?

If you convert your file to PDF--CS prints from PDFs regardless of what you upload, then you know what it looks like.

Actaully if you look at your source/original/working file, you should know what it looks like.  Have you printed it out (all or even a few pages) and trimmed the paper down to see how the pages/spreads look? You should print it out to proofread it.  And you should print out a few pages at least to see what they look like, these should be trimmed down to size.

There are a few forum member from the UK here, who can answer some of your tax questions.

Walton