Thanks guys for your responses, i have had the air suspension rear bags changed by a local garage, bought struts off ebay at £148 and £180 labour, car has now stopped dropping, i've had my suspicions it was the air struts since day one, my local Audi specialists wanted over a £1000 for parts and labour, granted chinese parts are hit or miss but the ones i've bought the seller asked me if the parts arrived ok and how did the fitment go, also they come with 2 year warranty, even if they only last 3 years i could change them again and i would of still saved money compared to Audi specialists or main dealers, original struts seem to last around 7 years.