I'm pretty convinced that the authorisation failures issue here is a card specific thing and nothing to do with the device.
After several more conversations with both Apple and Halifax with Apple continually stating that they cannot see anything wrong and there are no limits or blocks at their end and with Halifax continually stating that they aren't seeing the authorisation requests coming through, the latest Halifax conversation seemed to imply that Apple don't talk directly to the bank. The way is got explained - and I don't know how true this actually is - is that Apple send the request to Visa and it is then Visa who then send the request to the bank and then route the reply back to Apple, so it is just possible there is a block at Visa somewhere based on number / frequency of activation requests against a particular card which means that Apple are successfully sending the requests and the Halifax are genuinely not receiving them. This information came out of something that was almost mentioned in passing as the guy I spoke to at the Halifax referred to "not receiving the authorisation requests from Visa" rather than from Apple, but it is a possible explanation as to why neither Apple or the Halifax can see anything wrong at their ends.
Having tried and given up trying to find someone at Visa who would talk to me I eventually bit the bullet and ordered a replacement card from the Halifax. It arrived yesterday and hey presto, successful authorisation and added to my wallet on both my phone and my watch on the first attempts...
