All I can remember is one character, a mouse called Nils. I think he had a sword and was quite daring. And that’s it, I’m afraid- no idea of the plot, setting or any other details. I probably read it in the 1960s but I’ve no idea when it was written.
This is The Rescuers (1959) by Margery Sharp.
“a mouse called Nils”
Nils pulled his whiskers again. All mice have large families, and Nils was no better than any other man at keeping track of relations. (p. 48)
“I think he had a sword”
They left at once. (Nils just fetched his cutlass from the cloakroom, and he was ready too.) No one bothered to say good-by to them, in fact no one took any notice of them at all. (p. 42)
“and was quite daring”:
“Forgive me for joining you uninvited,” said Miss Bianca nervously, “but I am a delegate from the Prisoners’ Aid Society, seeking the bravest mouse in Norway, on behalf of a Norwegian poet imprisoned in our parts.” (p. 35)
The book was made into a 1977 film by Disney, but Nils does not appear: his role is merged with that of Bernard.