First reaction: I'd better Google that. Oh, that's not very helpful.
Second reaction: I should include 'area' in my search. Ah, that's better.
Discovery:Ooh, there's a UK Lakes Portal. It's a online collaboration between the Centre for Ecology & Hydrology, University College London and the UK conservation and environment agencies. It includes more than 40000 water bodies in England, Scotland, Wales, Northern Ireland and the Isle of Man with a surface area greater than one hectare. So it's pretty definitive.
Technicality: The list includes lakes, ponds and reservoirs.
Realisation: Ooh, you can zoom in and sort it by county. So...
London's 10 largest lakes
1)King George's Reservoir(163 hectares)
Also known as the King George V Reservoir. One of the two Chingford Reservoirs. Part of the Lea Valley Reservoir Chain. Located between Brimsdown and Ponders End (to the west) and Sewardstone and Chingford (to the east). Four miles long. Built between 1908 and 1912, and opened by the aforementioned monarch. Surrounded by a raised embankment, so cannot be seen at ground level. Eight metres deep, on average. Central puddle clay core. Divided into two compartments by an earth embankment. Limited public access. Permits available from Thames Water for birdwatching or sailing. Major wintering ground for wildfowl.
2)William Girling Reservoir(129 hectares)
The other one of the Chingford Reservoirs. Immediately to the south of King George's Reservoir. Part of the Lea Valley Reservoir Chain. Located between Ponders End and Edmonton (to the west) and Chingford (to the east). Three and a half miles long. Built between 1936 and 1951. Delayed by a) two dam failures in 1937 b) World War Two. Named after William Girling, Chairman of the Metropolitan Water Board in 1951. Surrounded by a raised embankment, so cannot be seen at ground level. Twelve metres deep, on average. No public access. One of London's principal winter gull roosts.
3)Brent Reservoir(49 hectares)
Located between Hendon and Wembley Park. On the boundary between Brent and Barnet. Built between 1836 and 1837. Formed by damming the River Brent, at its confluence with the Silk Stream. Water was needed to supply the Regent's Canal. Once covered 400 acres, but reduced to 195 acres in the 1890s, now 110 acres. Also known as the WelshHarp (after a former tavern on the Edgware Road). Popular leisure destination for Victorian picnickers. Now popular for sailing and other water sports.
4)Broadwater Lake(34 hectares)
Located in the Mid Colne Valley between Denham and Harefield. West of the Grand Union Canal, on the boundary with Buckinghamshire. One of several gravel pits, dredged in the 1960s, '70s and '80s. Nature reserve managed by the Hertfordshire and Middlesex Wildlife Trust. Contains several small islands.
5)Banbury Reservoir(28 hectares)
Another in the Lea Valley Reservoir Chain. Located just south of the North Circular, north of Walthamstow, adjacent to the Meridian Water development area. Completed in 1903. Approximately elliptical. No public access.
6)Lockwood Reservoir(27 hectares)
Largest of the Walthamstow Reservoirs. Part of the Lea Valley Reservoir Chain. Completed in 1903 by the East London Waterworks Company. Now owned by Thames Water. Eight metres deep, on average. No public access until 2017, but now open as part of the Walthamstow Wetlands. This is the northernmost reservoir (on the side of the road where the Visitor Centre isn't). Part of the rim is walkable.
7)The Serpentine(17 hectares)
Recreational lake in Hyde Park and Kensington Gardens. Created for Queen Caroline in 1730. Originally formed by damming the river Westbourne. Later fed from the Thames, now fed from three boreholes. Technically the western part is called the Long Water. One metre deep, on average. Contains a lido, supports pedalos, hosted the 2012 Olympic triathlon. Fronted by two restaurants and a Peter Pan statue. The largest lake most Londoners will have heard of.
8)Savay Lake(17 hectares) 9)unnamed lake(17 hectares)
Also in the Mid Colne Valley. Again flooded gravel pits. Technically seventh equal with the Serpentine. Located to either side of the Grand Union Canal, immediately to the north of the Chiltern mainline. Savay Lake lies west of the canal, adjacent to the Buckinghamshire border, and is well stocked with carp. The unnamed lake lies east of the canal, and is used for watersports by those at the Hillingdon Outdoor Activities Centre. HS2 will/should/may cross both lakes on a viaduct, to the annoyance of environmental campaigners. HS2 contractors are on site.
10)Ruislip Lido(15 hectares)
Reservoir in Ruislip Woods, north of Ruislip. Opened in 1811. Water was needed to feed the Grand Union Canal. Redeveloped as a lido in 1933, with beach and boats. Massively popular. Sailing ended in the 1970s when the water level was lowered to reduce the risk of flooding. Swimming no longer allowed. Beach remains. Part-circled by a narrow-gauge railway. Technically joint tenth with another of the Walthamstow Wetlands reservoirs.