central locking problem?

[matsmith]

hi guys, on my 2003 C200K sport coupe iv had a problem the past few days, some times the key does not open or lock the car, when it wont lock it i just have to put the key in the ignition and start it, and then take the key out and it usually works, when it wont open i obviously have to use the metal key

how would i know if the battery has gone? it flashes red when it actually works, so that means its fine doesnt it??

any more ideas or should i just go to merc dealers?

Thanks