Check this and produce the Milne in colour
On the Enclosure Map (page 107) the plots which were to make up the Collins Fortismere estate are centred round number 241, which already has two houses on it, and Firs on 238-9. The pond that concerns us is on plot 241. It seems clear that someone created the lake in Fortismere by extending this pond into a long waterway.
The 1822 Ordnance Survey map had the railways inserted in 1890, making it slightly difficult to read as all the details except the railway are of 1822. This confirms that there were already two houses close together, behind a hedge, on the Fortismere site and a third house, which was probably Firs, nearby. No drawings of any of these houses seem to have survived.