To expand on what Andy L said, the law states that you can carry a non-locking folding knife with a blade not exceeding three inches. Anything else is OK if:
1. You need it for your job.
2. It's part of a national costume. (Skein Dhu for example)
3. You have a reasonable excuse.
Point 3 allows some flexibility, so while it's illegal to carry a Leatherman Wave because of the locking blades, you're unlikely to get into trouble over it because the police know that people buy them as tools and not weapons.
The notable exceptions are automatic or gravity knives, balisongs, or "knives not readily detectable by conventional means, unless specifically for food use". This covers plastic knives (not toys!) or knives disguised to look like something else. All of these are illegal. If you own one from before they were illegal, you can usually keep it as long as it stays in your home.
It's also illegal to sell or market a knife as being suitable for combat, although sell the self-same piece of steel as a "survival knife" and you're fine. Go figure.
Chris