TKD wrote:

Occasionally a Tesco clerk would insist I provide a pin number, but when
I explained it was a credit transaction, not debit, they went ahead and
successfully completed the transaction.

It doesn't make any difference if its credit or debit, its if the card has
been CHIP and PIN enabled that you must use the PIN.

It's not quiet a "MUST". Since 14th Feb most/all UK card issuers are
refusing to allow shops to use the bypass pin option on their
terminals. Foreign banks may still alow pin bypass transactions to go
ahead but any shop performing a pin bypass is liable for fraud.

Have a look at
http://www.streamline.com/Already_a_...Qs/default.htm
for more details