My current Superb has 17" alloys, and my old Superb Comfort had 16" alloys.

However, my old VW Passat (2001 model) had 15" alloys. As the Superb was based on this Passat, would 15 inch steelies / alloys from a Passat of that era fit my 2005 Superb?

While I'm at it - would 16" alloys / steelies from a 2001-2004 Passat (B5.5?) fit the Superb I?

I'm after a set of cheapie wheels to stick some winter tyres on. Obviously a set of 15" or 16" tyres will be cheaper than 17" (I've got two sets of 17 inch alloys!!)

They should do, the running gear on the superb is the same as the 2001 passat.

Most Superb's come fitted with 16" wheels so they are no problem, i would of thought 15" would also fit as well as i can get a wheel brush between my brake calipers and the wheel.

2.5TDI and 2.8V6 for sure will not take 15in wheels - the brakes are too big (312mm)

Check the brake disc diameter - if they are 288mm you are OK with 15in, if they are 312mm then 16in wheels are required.