OK, after several tests and products traces and emails I have the answers from Lee at Shell:
Is the Shell Helix 5w40 I purchased from Eurocarparts genuine:
YES (Lee@Shell: Product codes were received and verified and ECP is on the list of UK stockists/suppliers of Shell Oil Products)
Is the Shell Helix 5w40 I purchased from Eurcarparts the same mix and construction as the dealer supplied 209L drums:
YES (Lee@Shell: Shell Helix Ultra 5W40 is a Shell grade, if the product in question is Shell Helix Ultra 5W40, then it will be the same product)
If a customer supplies Shell Helix 5w40 to be used by the servicing dealer, can the dealer decline to use it stating it is not the same as their 209L drums of the same product
YES & No (Lee@Shell: Its down to the discretion of the dealer but the oil, both customer or dealer supplied Shell Helix 5w40 is the same grade of oil so there is no reason for the dealer to refuse it)