I have a Lascal Kiddyguard retractable stairgate that I used in my previous house to keep little people out of the kitchen. Its major advantage is the flexible width. It is great for both extra wide gaps and very narrow ones that normal stairgates don't fit.
For full details and a picture, have a look at the John Lewis website:
http://www.johnlewis.com/230130957/Product.aspx
£30 - pick up from Putney (SW15).