Cheers for the reply veedub. I gave it a good flush yesterday, including removing the thermostat and reverse flushing. I think I've sort of fixed it, but am not sure how really! I noticed that on my golf the radiator overflow pipe has a constant fall on it down to the rad, but on the scirocco, it had been passed over the top of the main water channel output from the block so was above the level of the expansion tank. I took it off and lowered it so it was the same path as the golf and bled what air there was in the top of the rad by taking the overflow pipe off until water flowed out. It now seems a pretty good flow. Hopefully that's sorted it!