An excellent link asdf99.
As an ex trading standards advisor I can confirm that we used to use this service as well, when people called.
Most people can settle for the free search (found on right hand side of the screen).
Some things to look out for include:
The date of incorporation (how long has the company been going).
Are the accounts up to date?
How often has it changed its name?
Does the companies contact details match their website details? (Although they do sometimes differ).
Is it in Liquadation?
Finally if you cannot find the (UK) company anywhere on the site, don'y buy from them. They are either very small companies (you're in trouble if they go to the wall) or potentially fraudulent
If there is 'anything' that gives you pause for thought, look elsewhere. There are plenty of reputable companies out there.