Ahhh.... but there was more than one circle at Hanworth!... and I quite agree the one in the picture you have linked to is in a completely different part of the airfield.
The one I linked to is clearly to be seen in the picture on page 87 of the book referred to earlier in the post ('Coming in to Land' by Tim Sherwood. A Short History of Hounslow, Hanworth and Heston Aeroromes 1911-1946. Heritage Publications. ISBN 1899144307. £11.99. Available from Hounslow Library.)
The photograph shows a circle in white, to the left of an imaginary straight line drawn between the point where Park Road joins Hounslow Road and the "the house" on the island in the centre. This can be easily transposed onto the Flash earth image.
The "Hounslow Circle" can also be clearly seen with a letter "T" next to it in the picture on page 17 of the same publication.
Unfortunately I do not have a scanner (and I’m not sure how legal it would be) so I can’t post the pictures.....