Really it is a personal thing.
If you have experience of HTML and CSS etc then there's no reason why you shouldn't try and do the website yourself.
Some webhosting companies like
1&1 Internet - Home have an online website builder available for you to use a basic template and alter it to suit yourself. This usually just involves clicking on some text and changing it or changing pictures etc.
There are plenty of easy to use website building programs you can use as well. A lot of them are free and they work by wysiwyg (what you see is what you get) That really means if you stick an orange box in the middle of the page and put some text in it, that's what you'll get when you publish the web page.
You can also download free website templates which will give you a starting point. Using an HTML editor, you can move things around, change text and pictures etc.
Basically there are pros and cons to whatever you choose. If you get a web designer to do it for you, the cost could be anything from £200 for a simple two page website up to £20,000 if it's got all the bells and whistles.
You could try dipping your toes in the water by using facebook, google+ or twitter if you're just wanting your company to have a web presence. See how that works out and take it from there.
If you do decide to set up a website you have to remember you'll need to buy the domain name (
www.yourcompany.com), webhosting (your space on the internet), possibly email service as well (info@yourcompany.com). You'll need to optimise your website to help make sure people will be able to find it when they search for you. When someone searches on google, they don't always search by company name. They'll probably search for scaffolding company in whatever place. You need your website optimised so it shows up in the search results.
Don't forget that you'll have to renew the domain name every 1-2 years. It's not yours forever and if you forget to renew it someone could take it.
Sorry if I've gone on a bit mate, just trying to give you the ups and downs
