According to the BBC an Ofcom report has shown that over a third of adults use smartphones and that there has been a dramatic rist in mobile internet use.
So, make sure that your website works OK on a mobile device by trying it on a mobile device (!) or by jumping into your Google Analytics and looking at Visitors | Mobile | Mobile Devices. Check the average pages per visit and average time on site per visit. These will be lower than the figures you see for pages accessed by a ‘proper computer’ (atlhough iPad figures will be higher because of the more readable screen size), however if these figure are very low then it indicates a problem on a given device.
There are a few intermediate steps between a site designed for a desktop and a hand crafted mobile site (or indeed a mobile app), but at the very least your site should display well and function correctly.