Have you looked at the one thats retractable it comes with different attachments including one that straps round a bannister.
http://www.amazon.co.uk/Lascal-Kiddy.../dp/B00009SS6B (http://www.amazon.co.uk/gp/product/B00009SS6B/ref=as_li_ss_tl?ie=UTF8&tag=childminding-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B00009SS6B">Lascal KiddyGuard Safety Gate</a><img src="http://www.assoc-amazon.co.uk/e/ir?t=&l=as2&o=2&a=B00009SS6B" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;)
We have this one cos we have the same issue, as well as a bizarre bottom step. It does not have to be fitted in the same way as a standard gate, as in a straight line. So you could put the cassette on the door frame rather than the side of the stairs, so the handrail is not an issue at all, then close the gate across to the other side of the stairs. It is a pricey stair gate, but I actually find I prefer it because it rolls away when not in use rather than being permanently fixed and in the way. It folds up small and totally out of the way so you can forget it's there at weekends and evenings. I think it would definitely work on your stairs.
The alternative is get your hubby to attach a strip of 2x2 under the handrail to extend your wall out to the same level as your handrail and attach the gate to that - we've done that before too, but it can be a problem if the plaster is not great quality or if you have to remove it when you move out.