Thanks for your replies. Some good advice, although as adequate as the Super Sports are in the wet, I might skip the torrential rain
Stripping back to bare metal seems a bit extreme and expensive but would no doubt give a great result.
I share the concern that adding any protection could hide underlying rust.
I try to keep the car off the road as much as possible in winter but would be ideal not to drive it at all as suggested.
Jrclassics are fully booked until October so if I do get them to do any work, it won't be for a while. Based on the advice from you guys, I'm reluctant to have all of the bottom of the car treated. The cavity wax protection they offer sounds like a smart idea though, so that's something I might pursue.
I'll start with replacing the rusty bits and take things from there. Also considering stainless exhaust clamps. These seem good value at £25:
http://www.ebay.co.uk/itm/BMW-E46-M3...IAAOSwHxVW69F2
And a more expensive option at £60:
http://www.ebay.co.uk/itm/stainless-...wAAOSwZ8ZW2JDY
Would welcome any thoughts on whether either of these are a good option or whether there are any better solutions?