Car alarm locking sensor

Does anyone have any up to date advice please on a locking-unlocking issue with a C240 Mercedes. I can now only get into the car which is in my own garage using the key. Central locking and unlocking does not work. Initially last Saturday at 1am a possible low voltage or burglar intereference triggered the hazard warning lights which would not stop after re-setting several times, so overnight battery went flat by 8am. RAC came out and tested battery from a spare in his van. Battery, he said, was fine. He took out one fuse from boot fusebox and suggested I had an alarm problem. But when he charged up the battery a "Visit Workshop" warning message showed on dashboard. New Bosch battery is £130 from Halfards but RAC said I didn't need one. Now can only open car using the steel key. Mercedes dealer said it needs to go to them - but that starts clocking up £100 plus an hour just for labour. Any precise suggestions please. GB

 

Hello welcome to the forum, Apart from asking how old are the batteries in your key fob? as I have read post's about these causing problems, I'am afraid I cannot help much more but I'm sure someone will soon be along who can