Just been notified of this.
https://omaze.co.uk/pages/london-vii
I'm having difficulty finding its precise location as there are so many similar houses but I bet some genius on here will succeed. I gather that the Mapesbury Conservation Area consists of these roads - Anson (Broadway to Chichele Road), Dartmouth, Dawlish, Exeter, Keyes, Hoveden, Lydford & Mapesbury (to the railway), Teignmouth, Walm Lane (including side from the station to Broadway).
Definitely not worth £3.5 million and the attic bedrooms could only ever be used for occasional visitors to my mind.
Still, you'd get well over 2 million if you sold it which is a chunk of money even after fees.