Immobiliser headaches

My 1999 Renault Laguna has had to be towed to a Renault garage because my immobiliser wouldn't work therefore rendering my car completely useless. Upon phoning today and having been lead to believe the cost would be between £55 and £135 to have the key recoded and reset, I have been told it will cost £360.41. This is for an unsuccessful 3 hour diagnosis and the cost of ordering and fitting a new "plith"??? key and a decoder. What's this all about?????