Satellite navigation devices (sat-navs) have taken Britain by storm. In the last two years, sales of portable sat-navs have increased fivefold and there are now more than four million 1 systems in use on the UK’s roads.

As their popularity has increased, prices have tumbled. While early models cost £1,000 or more, basic models can now be found for less than £100 at supermarkets.

Which? has awarded six sat-nav systems Best Buy status. The best of the bunch was the Snooper Syrius UK & Ireland (90%, £286), which experts thought had “everything a driver needs.” Testers were impressed by its accuracy, describing its handling of the tricky city route as “perfect”.

Close behind is TomTom’s mid-range Go 510 (89%, £269). The impressive software lets drivers select a route according to a range of criteria, such as the quickest, the shortest or one that avoids toll roads.

Third place goes to the Garmin Nuvi (88%, £190), which was simple to set up and “a doddle” to use. Searching for destinations was especially easy thanks to a seven-digit postcode search function.

Other Best Buys were the TomTom One UK (86%, £180); TomTom Go 910 (86%, £350) and the Road Angel Nav 6000 (81%, £180).

Richard Headland, motoring editor, Which?, said: “Sat-navs often get a bad press for sending drivers the wrong way or down unsuitable roads, but our tests show that many of the newest portable sat-navs work very well.”

“They’re also much cheaper to buy and easier to keep up to date than factory-fitted systems. If you want sat-nav in your car, make sure you go for one of our Best Buys.”

But police forces around the country have warned motorists about increasing theft of portable sat-nav systems from cars. The demand for black-market systems and the smash & grab ease at which they can be stolen has led to a big increase of car break-ins.

Tell-tale ring-marks left on windscreens and empty cradles on the dashboard are advising thieves that a sat-nav system in use and may be stored in the glove compartment. Break-ins are so easy, and criminals don’t care about the damage they cause that just a suggestion a sat-nav might be in the car, is enough to instigate a crime, say police, leaving motorists with a car that is vulnerable to rain and further theft.