The really strong stuff is 0.7mm +. Steel sheeting makes a huge difference to the 'stick'.
They also do Neodymium magnets (unbelieveably strong ones) in any size you want.
You can check them out yourself at www.magnetick.co.uk.
They are part of a company called Indigo (logo 'A whole world of industrial materials') who you can find at www.indigoplc.com. Indigo(who I suspect are apart of a much large outfit) sell lots of other useful stuff too.