To the untrained eye, it looks like some image from the television spy thriller Spooks – an aerial view of houses and back gardens, with roads, cul-de-sacs marked out by blocks of yellow and orange colour.

For the last few days at Harrow Council, we have been poring over thermal image pictures taken by a plane we commissioned to fly over the borough one icy night in January. Climbing to more than 3,000ft, the plane spent three hours criss-crossing the skies above Harrow, taking a series of images of the borough with a camera specially fitted to detect heat.

So why did we do this? Let me reassure the advocates of civil liberties that this is not some sinister departure into Big Brother surveillance; the equivalent of CCTV with wings taking a prurient peek into the bedrooms and private lives of our citizens. The purpose of this exercise was to understand who is living in our borough, and whether they have the right to do so.

The airborne mission – which took place in January - is in fact the latest stage in the council’s campaign against ‘beds in sheds’; the growing trend for standalone mini bungalows springing up in the back gardens of our cities and suburbs. Unscrupulous landlords like them because they squeeze easy money from tenants desperate to live in the city.

We dislike them because they are exploitative, illegal, and cram people into properties that are frequently fire traps. These secret citizens also cost their neighbours because they generate rubbish and use council services – for which they are frequently not contributing.

Our previous best guess was there were 75 beds in sheds developments in Harrow – a figure we based on complaints made by neighbours to us (who can be upset by everything from noise, excessive car comings and goings and rubbish bags dumped out front).

As we studied the thermal image pictures, a starkly different story emerged. A suspect development shows up on these kinds of pictures as a rectangle of yellow light, generally because these developments are constructed quickly without proper insulation. By contrast, a house looks like a dark square from above, with a halo yellow glow around it coming from windows.

Experts at the council have been comparing these tell-tale heat signatures of yellow against earlier daytime maps of Harrow. This shows us where buildings we didn’t know about (i.e. that don’t have planning permission) have sprung up.

The result is that that we have found 319 suspect developments in Harrow – four times the figure we initially thought. Now, not all of these will be Rachman-type beds in sheds – we have so far found five cannabis factories, for example, whose constant high-temperature lamps were given away to the thermal eye in the sky. We have also found one or two houses which are poorly insulated, and we have been able to help with insulation there.

However, there are dozens upon dozens of suspect buildings our planning officers will be visiting over the coming weeks, some of which we already have anecdotal intelligence about. The thermal image flight has proven, beyond doubt, that beds in sheds is a widespread issue across London. If our corner of North West London has revealed several hundred of these instant illegal homes, how many more have sprung up across the capital?

Our dispute is not with the people who live in these places; they are frequently at the bottom of a pyramid of exploitation. In once case in Harrow, we found a house where three people were paying £55 each for a mattress in one room. We came across another overcrowded three-bedroom semi detached house with illegal outbuilding which we estimated was capable of making the landlord £4,000 a month.

It is manifestly unfair that people living next to these developments should, by paying their council tax, effectively subsidise their neighbours’ council services. We are not the only public sector organisation to have concerns. For the fire brigade, there is this dilemma: if the hidden garden bed-in-shed development burns down, how will firefighters get to people if we don’t know they are there.

Unfair, unsafe and unsustainable in a capital city where population density in parts is reaching saturation point, beds in sheds is a serious national issue.

Here in Harrow, we will continue to campaign on this and confront landlords fuelling the problem. Our thermal image pictures have allowed us to get a more detailed picture of how extensive this problem is. The only way we have been able to understand what is really happening on the ground has been to take to the sky.

Cllr Susan Hall is leader of Harrow Council