Well, many apologies for the delay in reporting back, but I've now had the chance to try all three of the suggested free downloads. The good news is that they will all merge and split PDF files very effectively.
However, there was one fact that I foolishly omitted from my original description of my needs - that is, the fact that the PDF's I want to concatenate are filled-in PDF forms. Only one of the packages seemed able to handle these, and it was PDF Sam (Sam = Split And Merge). This has a special check-box to click if the PDF's are forms, and it preserves the forms perfectly.
So, huge thanks to everyone for their advice and help. Job done!
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