Finally resolved it:
Credit cards and debit cards must be issued in the country where your PayPal address is registered before you can link them to your PayPal account. If you use a UK card against a UK account and are on a computer in China or on a VPN then alarm bells are liable to go off in PayPal’s security office.
China Construction Bank supplied a UnionPay debit card which now works. You need to upgrade the China Construction Bank debit card before you can use it so that it can be used on the internet. For a week or so PayPal refused this card as being an “Invalid debit or credit card type” as they did with every other card I presented. However, it seems that this is not necessarily a problem with the card but may well be with the flaky connection between China and the rest of the world. Perseverance is advised and the signal is typically better in the morning than the evening.
The UnionPay debit card from China Minsheng Bank did not work with PayPal. The Bank of China took about a hour before they gave up trying to open an account for me because of “system failure”.