The Gothic Ruins on the east side of the Gunnersbury Park, just off the North Circular. These ruins are the left overs of the stables and include battlements, etc.
Gunnersbury Park is a park in the Brentford ward of the London Borough of Hounslow, in west London. Purchased for the nation from the Rothschild family, it was opened to the public by Neville Chamberlain, then Minister of Health, on 21 May 1926. The park is currently jointly managed by Ealing and Hounslow borough councils. See here