travelling from France to UK with DS1 & 2 who don't have my surname

[AuldAlliance]

Tomorrow I am flying from France to the UK with DS1 & 2.

They have their father's surname, not mine.

As a precaution, I was advised to have a letter from DH authorising me to travel with them.
Is this right?
What do I need?
Just a signed letter? (which I could have written myself)
A photocopy of our livret de famille as well, or of DH's ID?

TIA

Currently it's only a problem if it's the name that's different though. It could well become the case in the future that permission of the other parent is required but at the moment having the same name is usually sufficient. If you don't have the same name as your DCs then you're more likely to have problems, so bunjies is in a less sticky situation than auldalliance. At the moment.

...unless you go to the US or Canada or some Italian airports.

My ds has the same name as his father, but when he took him to Canada, they let him in, but said he should have a letter of permission from me, and that the US border control would not allow him to take ds there without letter (in my case an email was deemed sufficient).

However in Spain they didn't give two hoots...

That's a relief.