How can I go back to previous version of software as iOS 5 has stopped car bluetooth working. iPhone 4

I have an i-phone 3GS and a 56 Plate Ford Fiesta with factory fitted bluetooth audio.  This is a Nokia bluetooth but has allowed me to call hands free with this phone and previous i-phones and other brands with no problem.  I upgraded to iOS5 and I can now not make calls through it.  If I press call, the radio says 'dialling' but the ringing tone and when someone answers just goes straight back to the handset and not through hands free which makes it redundant in the UK.  I rang Apple and asked how to go back to a previous version of software that does work but was told Apple staff were not allowed to tell me how to do this.  His only other suggestion was that I buy a new car which wasn't massively helpful or practical.  I have seen quite a few messages on the internet of people having this sort of problem after they upgraded.  Does anyone know how to go back a previous version or how to get bluetooth back working with my car as I quite like the iOS 5 update it's just no good if I can't use it in car.? I've tried deleting and re-pairing but that hasn't worked.  Does anyone also know whether Apple have picked up on this bluetooth compatibility problem and whether they are planning on doing anything to fix it as it does seem to have affected a lot of people and if it can't be resolved I'll just have to stop using i-phones and go to another manufacturer as it's not helpful living in a country where you to have hands free and then not being able to do that because a software update prevents it.  Any help would be greatly received.