Thank you to everyone for their very helpful replies. I think you have provided the answer as last week I washed the car including hosing under the wheel arches just prior to putting it into the garage. In future I wil certainly do as suggested and take the car for a short drive to dry off the brakes before putting to bed. I will also leave the brake off when in the garage. I am also greatful to you for putting my mind at rest that it is unlikely to be anything serious.
What an amazing forum TM is, an answer from so many helpful people only one hour after asking the question.
