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Buses and taxis normally deposit you at the Parkinson building - big white building with a tall clock tower, columns and steps in front - this is what it looks like (from an e-postcard!):
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Also see annotated Google map of the campus with the Edward Boyle Library highlighted.
If coming by car, come to the main entrance on Woodhouse Lane (by the Parkinson building), and follow the signs for the Visitor car park. The charge is £5. It can sometimes fill up, but as far as we know there are no big events on 25th June!