A sample of results in the 2001 General election shows that turnout is inversely proportional to majority. See figure here http://www.greenhealth.org.uk/Democracy.htm . Turnout falls from 60% to 50% if the majority rises from 4000 to 1600. In other words, some people realise there is no chance of their vote making a blind bit of difference. So if we want to increase participation and turnout, we need to go for PR.