The legalities of "Win a Free..." competitions

[Hevside]

Hello

We are planning to place a local newspaper advert for a car air-conditioning service and want to offer a free service to the winner and runners-up prizes of free air-con health checks. It is a genuine offer and will of course, hopefully, produce a database of interested customers and work generated by the free health checks.

Can someone please tell me if there are any legal wording or rules regarding this type of advert so that we don't get it wrong.

Thanks

Heather