I'm with you matey - I had a Q5 and Watford said that because I went indie for service they must've removed this cap stopping the washer fluid as it was draining out as soon as I topped up. They told me this would involve removing the front bumper; I find it hard to believe that the indie (a very reputable VAG specialist in St Albans) in the few hours they had my car, removed the front bumper just to remove this cap!
In the end they agreed to reduce the charge (I thought it was covered under manufacturer warranty so told them to go ahead) to one hour labour 'only' so cost me around £150 to get a plastic cap put on!
By the way, my current issue isn't with them as I went elsewhere after this 'lovely' experience at Watford, even though it's 5 mins from my house...