Thanks for all your wonderful replies...but I know it will be a long long way to go with Willie and maybe he never trusts, but I hope he'll be happy around here...
There's been a young black/white cat in an amazing plight few years ago around here, covered in ticks and very thin...I've informed the local animal organisation, they've tried to trap him, but they've failed and ever since this cat was gone... ...until this summer, when suddenly Willie (as we named him back then) appeared again in our garden...
First I wasn't sure wether these two cats are the same, but I am now quite sure, although this cat now looks quite well fed (he's a keen hunter and they are fed at a feeding station close by) )...back in the summer I've phoned in to the local animal organization again to hear about the strays in our area and they said all have been trapped/altered/released...
But now (as he comes closer to the windows) I think I've seen something hidden under Willie's tail ...I am now pretty sure he's not neutered...
I'll phone again with the organisation to get a trap as soon as he trusts us more and as soon as the weather will be fine again, because they can't board him anywhere after surgery and he would surely freak out when he wouldn't be released quickly...

I think he very much enjoys our garden and it's funny that he and Nellie have certain time schedules who's turn it is to roam the garden...if they meet they just curiously look at each other and go their way...

I hope everything works out fine and we won't fail again trapping him...please keep him in your thoughts for a happy ending!!!
Thanks...