I own a professional web design company and would agree with what amazingtrade has said.
Much like any other service industry, everyone knows someone down the pub who's brother's daughter is studying the subject at college and will be 'prepared' to do it for £150 and a bottle of wine but if you're in any way looking at doing things properly, you should use a professional company.
I recently replaced all the carpets in my house and when I was told how much the fitting would cost I *considered* doing it myself for a second. Now that the carpets are down and look amazing, I'm very glad that I did use the professionals and didn't go the false economy route.
And when I say use a company, I really do mean more than one person; and a company with actual resources and trackrecord. Most one-man-band operations tend to be either techies with a loose grasp of design techniques or designers with a basic grasp of coding skills. To do the job properly you need a current, strong suite of skills that encompass both aspects, plus knowledge of SEO and current Internet law.
Any decent company should communicate with you and display an understanding of what you're trying to achieve with your site, provide you with a fixed price quote before work begins, should not expect full payment in advance (although a small deposit actually works better for both parties), a scope of works/contract (that protects both parties) and should be able to provide on-going support for their work (as set out in the contract).
Of course, the best way to do your homework on any professional web design company is to check their client portfolio and to phone two or three of those companies and obtain their unbiased view of the service and product they received.
As a very rough price guide, £500 to £1000 should buy you a level of site that's actually worth having and be of a level of design that should not turn users off. £1000 to £2000 should buy you a larger or more advanced site, possibly featuring a content management system of some kind. £2000+ gets you into e-commerce, database and more complex sites.