UK RHD C250 1998 airbag light & impact sensor problem

Hello all,
car just back from the dealer. i had it in to diagnose red SRS light staying on.
history is that i had car valeted and shortly afterwards the SRS light came
on. they absolutely soaked the carpets and seats. i hoped it would be ok once dried out, but not the case. the valet firm has since gone out of business.
i took the car to a local independent garage who cleared out some logged fault codes (which didn't return) but said that a new code was constantly being logged even after cleardown relating to the airbag - advised me to take it to the dealer since they couldn't diagnose any further. they said it related to a drivers side sensor.
the dealer report says "carried out diagnostic on srs systems, found fault with drivers side impact sensor. please note that there are signs of water ingress at the drivers side of the vehicle. the impact sensor will require to be replaced after the vehicle is checked for water ingress and rectifications carried out".
estimated cost 600 UKP!
the only water ingress will have been from the valet.
on the phone they told me that the sensor was bolted to the floor under the seat.
i'm fairly handy technically, so i thought i'd get the sensor out and have a look - maybe buy it direct from the dealer, replace myself and save .
for some reason, i had a bit of brain fade and removed the PASSENGER seat, carpet and took out the first sensor i found (obviously i took the recommended precautions i had googled - disconnect battery, stay well out of the way, etc).
the sensor i removed said "TEMIC MRSA SIDEBAG" and did look like it had signs of water damage.
my question now is, should i remove the driver's seat? how many sensors are there in the passenger cabin, bolted to the floor?
also, will i have to get this code finally cleared in order for the light to go out if i actually manage to fix the main problem?
all help really appreciated!
many thanks in advance paul

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