I read an article on the BBC site by a company that runs online e-commerce websites which clamed that having spelling errors in youre online shop can damage conversion rates by as much as 50%. I was not convinced of their case, believing it to be a link-bait PR article by the company in question to make you go and visit their website. PC Pro have posted a simlar article on their website stating that spell checking can boost your web profits (allbeit using the same company as the BBC website).
There's no doubt that missing images, poor spelling, inappropriate navigation systems, layout errors and slow page render speeds all play a part in your conversion rates, but most people don't actually spot spelling mistakes on web pages. There are 4 deliberate spelling mistakes in the first paragraph - did you notice them? As long as the errors are not essential parts of the page like 'add to shooping cart' or 'chekout now' its unlikely to have as much impact as Mr Dumcombe claims. However there is no independent evidence to compare with the findings. I don't think many online shops would be happy to risk losing money just to find out how much money they would lost by having spelling errors on their website.
If you're looking at Conversion Rate Optimisation, There's a great article on strategies to improve conversion rates here, and I have also written an article about e-commerce conversion rates and how to improve them here: http://www.mykeblack.com/blog/ecommerce/7-ways-to-boost-ecommerce-revenue.