2004 Renault Clio

Electrical problem
2004 Renault Clio Manual

I've got a Renault Clio and have never had any problem starting it. Last week I took it to a garage to have some rear damage repaired and since repairing the bodywork the garage now tell me that the car does not start. They say that the battery is completely dead and cannot be jump started and that the relays are severely corroded. They are therefore suggesting that I need to fork out £180 for a new battery and 4 relays. I'm abit sceptical about their diagnosis because I've never had any problems starting the car and it only had a full service and MOT 5 months ago. Is it right that a problem like this can just come out of the blue or should I be suspiscious that something has gone wrong with in the repair process? Thank you for you help in advance. I know very little about cars