Apple Pay not working with Mac Safari on websites

I have:

* An iMac running Mojave 10.14.6

* A Apple Watch 4 fully updated

* IPhone 7 fully updated.

Up until recently, I have been able to make Apple Pay purchases very happily on my Mac. Then they started failing about (1 month?) ago, possibly after a MacOS update.

The symptoms are:

1. A dialogue from Safari saying that it cannot find my watch ("make sure bluetooth is turned on")
2. If I turn off Allow Payments on Mac on the Watch, I get instructions to add cards to my iPhone wallet.

The same transaction works fine if I buy the item on the phone using Apple Pay.

I have tried.

1. Toggling 'Allow Pay Payment on Mac' on both Watch and Phone
2. Restarting both devices
3. Removing all cards from both devices and then re-adding them.
4. Taking off Content Blockers on the website on the Mac

Any other ideas?