I have just solved this my self and was simple to do. I logged in to my router clicked, on wireless settings under the set up category, and where it says channel there is a drop down box with Auto (which mine was set on) and channels listed 1 to 13. I selected 3 (not really sure why 3)and then apply, then it did its thing of connecting and hey presto he has a good connection. So hopefully this seems to have worked, but will monitor it over the next few days to make sure it has improved.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been