There's been an upsurge in secondhand cars being clocked, which can be dangerous, and is basically fraud.
There's calls for the EU proposed ban on this activity to be retain under law in the UK, but until such a thing comes to pass, there's some things you can do yourself to make sure you're buying safely.
The bank, TSB, as set up a good guide and checklist for those looking to buy a secondhand vehicle, which you can see here.
Some of the more obvious tips is to make sure you always take a car for a test drive before you buy it, and have a poke around the engine first. If you don't know what you're looking for, take someone with you who does.
Another good trick is to arrive a bit early - some snide sellers will warm a car up in advance, so it runs better.
If you're early, you can catch them at it. Make sure the engine is cold before you start the car.
You should also use the Government website to check a car's MOT history using the registration plate.
Make sure the vendor has the car's V5C registration certificate, and that it matches the seller's ID.
If you're buying a car with cash, be sure to make a small payment with your credit card too, even if it is only £1.
What this does, is provides you with protection under Section 75 of the Consumer Credit Act. That basically means the credit card company are liable if the car isn't as described or knackered.
Also, do a logbook loan check (or HPI check if you prefer), which you can sort out for £19.99 at hpicheck.com.
That offers you cover of (up to) £30,000, and will inform you if the car you're buying has any outstanding finance on it, or if it has been recorded as stolen or an insurance write-off.
That saves you from having your car repossessed, when you've done nothing wrong.
Keep your wits about you, be thorough, and you should be just fine.