Found this in Tesco Hoover Greenford when I went to get a smoke alarm thanks to
starskytheman. I picked up one up as it's advertised as a portable one so can take on holiday etc so extra piece of mind for the kids.
Showing £29.89 online
They also had the non digital one for £12.49http://direct.tesco.com/q/R.200-3014.aspx