Update: I just found this post from Macropod which looks like the basis of an answer:
https://answers.microsoft.com/en-us/...f-8642e46fa103
I did some tweaking with the data and made it relational, then followed the post, so I've got the merge itself working now.
Unfortunately I'm not able to control the format of the data in the usual way (adding
\# "£0.00" to the MERGEFIELD) as it uses sql-type fieldnames, and Word uses (example) £1.9 instead of £1.90 if I try to format as currency in the xlsx data source.
Can anyone suggest a way to tackle this issue?