Touareg Brake Assist warning

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Thoughts on the foolowing appreciated. We have an '05 Touareg 2.5tdi and twice in the last 8 months we've had a 'Brake Assist refer to workshop warning' come on, either immediately on starting the car or shortly after starting driving. The car was taken to the dealer on both occasions and was returned working with a report of items that had been replaced/repaired. However, the interesting bit is that on both occasions tyres have been renewed but nothing said to us, 4 new ones last time, 6 in total since January, that must be nearly £1000 of tyres! I'm very suspicious that something is fundamentally wrong with the transmission/differentials and the dealer is covering it up by fitting new tyres to equalise rolling circumference. So, has anyone experienced anything similar? Am I paranoid?