I had a very similar situation to this when I bought my BM.
Taking my boy to school one morning (about 2-3 weeks after buying the car) in heavy rain and the car stopped suddenly with DSC, DTC, Gearbox errors flashing up all over the dash, turned out to be a problem with the ECU housing being blocked up at the bottom and was basically filling up with water. Diagnosis was a water damaged DME ECU. Cost @ £1,137.
The warranty company did what most do and gave some lame excuse about how electrics were not covered for water damage. I went back to the dealer that I bought the car from with the issue and they made every effort to distance themselves from any responsibility.
I researched on the net quite vigorously about my situation and 'trading standards' and 'fit for purpose' was the common theme I was picking up.
I got onto a site (can't remember it) and downloaded a trading standards template (it may have been from Trading Standards site come to think of it) and you basically fill out the unique info like date etc and send it off to whoever you need to. I did this and sent it to the car dealership I bought from and followed it up with a call to which they complied, stepped forward and payed the bill.
If you have time (and assuming you haven't already sorted it) check it out.
Good luck bud
