This may help others diagnose the problem.
I saw a comment somewhere about using an Android app called [link|https://play.google.com/store/apps/details?id=com.ssaurel.nfcreader&hl=en_GB&gl=US|NFC Reader|new_window=true]. It has an option to "Scan a Tag", which, once tapped, will display NFC information about the device/card presented to it. That seems to work with both the bottom and top edges for the GW4.
I've noticed that sometimes I have to continue to hold the watch to a reader in a shop and the phone buzzes twice: the first attempt is rejected and the second accepted. I just did the same with NFC Reader app. The first attempt shows a lot of strange characters but the second is parsed correctly by the NFC Reader app. The GW4 NFC technology is shown to be "IsoDep, NfcB". I assume there are different signal types/standard?
That may explain why some readers say present one card only when I use the watch to make payment. From searching, this seems to be a firmware fault in those readers. My guess is the watch is sending two NFC signals to readers to allow for different standards?