Just because someone is at work – doesn’t mean they shouldn’t count towards web-usage stats though, does it? If you want your site to be accessible to large corporates – you still need to design for IE6.
Or even if you want your site to be attractive to those who are browsing at lunchtime – you can’t sneeze at a 12% market share.
Of course, given your browser statistics, it may be that Conservatives are less conservative when it comes to upgrading their browser. Perhaps a bigger and more neutral site – say BBC news or The Sun – would be a better benchmark?