SureAI

I'm baffled, I can't catch this chap at all. I sometimes get near him and have tried engaging him with the spacebar and it doen't seem to work. Casting or shooting arrows at him don't work either.

help appreciated

You're not supposed to interact with him, you just need to follow him long enough so that he gives up running from you and face you. Its tricky to keep up because its dark and you can easily lose him up on the battlements, when he dives into the river. Just keep on following him until he faces you.

Still don't get it , he stops sometime and will stand there forever but as soon as I approach he's of again 'til he jumps off the battlements.

you have to follow him longer.

the chace is goint through komplete Erothin.

when he jumps from the wall into the rive you have to follow him

he will stops and give up, if i remember correct, in the sewers

Thanks got it

I was losing him when he jumped in the water and had got the idea you had to catch him before then

I found that if I entered the house and attacked him quick enough and knocked him out, I could pick pocket the letter off him.
He is a 'main character' as he appears later on in the story (he is also one of the trainers/sellers you will find in the sewers), but knocking him out did not affect me buying things from him, or him interacting with me later on.