Glyn,
You're smarter than that... Usually when someone isn't a regular poster/participant in this forum, they don't include all, or most of the necessary details in their initial posts... Hence the reason for my suggestion... And as you can see I hope, that the OP does mention two key words in his follow up... Thickness of material - "28mm..." "Metallurgical & Charpy..." And "longitudinal line pipe manifacturing..." As a result js55 was able to provide his perspective & opinion based on the many years of experience js55 shares with all of us in here...
I would only add that depending on the location where the welding is going on, it might be prudent to perform a minimal preheat to metal that's 28mm thick prior to welding.
Respectfully,
Henry