HOW TO KEEP THEM UNDER A WATCHFUL EYE

Bluetooth devices

These are cheaper devices that rely not on SIM cards but technology that has a range of up to a few dozen yards. Batteries can last a year or more. The best known is probably Tile. Its tracker, Tile Sport, has a range of 200ft and costs £39.99 for two.

GPS devices

GPS trackers rely on a SIM card, which means you will have ongoing monthly costs. They can track a subject anywhere, including abroad, but are far less accurate and drain battery power. A range of devices, from insoles to watches and belt buckles, are available at Techsilver. co.uk and trackershopuk.com, including these:

The Gator, £99, plus sub from £9 a month

techsixtyfour. com A groovy watch (above) with a preinstalled SIM that works on all networks.

Track Your GPS tracker, £124.99, plus SIM card trackyour. co.uk

pendant tracker with a battery life of 100 hours.

Weenect, £79.99, plus sub from £3.20 a month

A Frenchmade tracker (left) with an emergency button for voice calls.