Councillors get all kinds of questions from members of the public. Believe me, you don’t want to know some of them. But while councillors take most in their stride, I was taken aback by one posed at the recent South End Green Festival.

A man came over from another political party's stall and after a few seconds looking at the Conservatives' stand highlighting some local issues the local councillors were working on, asked: "What's wrong with building on Hampstead Heath?"

Now, I'll tell you - in my time representing Hampstead, that's a new one! The Heath was what attracted me - and I'm sure many readers - to the area. It defines our area and offers relief from the pressures of city life.

This gentleman wasn't talking about anything as drastic as building a motorway across South End Green - the absurd plan that contributed to the first ever South End Green Festival. Instead, it was about small incursions, and whether it's right to fight against them.

Well, for me, it's a red line. Hampstead Heath is protected as Metropolitan Open Land - a status roughly equivalent to green belt, but within London's built-up area, including the Royal Parks, Primrose Hill, Highgate Wood, and Fortune Green.

It's the strongest protection of land available in law, to protect London's lungs, and if it can't be protected, what can? That means it should be fought tooth and nail to be preserved, even when it's "only" being nibbled around the edges.

Yet because of those nibbles, across London, 35 acres of Metropolitan Open Land - or about 20 football pitches - are lost to development every year.

The Heath has fared better than many other sites - primarily because of the "Avengers" arrayed in its defence: the Heath & Hampstead Society above all, but the Highgate Society, the Hampstead Forum, our Conservation Area committees, the City of London Corporation itself, and many else besides.

Its technical status as Metropolitan Open Land is invaluable, yet challenged and contested surprisingly often. A third of all appeals against Camden Council heard by the Planning Inspectorate this year have related to Hampstead Heath.

One appeal was withdrawn last week, but two more threats are coming in Hampstead's Vale of Health this autumn. One, to be heard in September, concerns demolishing a "chalet" that was unlawfully built on the South Fairground site, while another will reconvene in October on whether the North Fairground site can be lawfully used for permanent homes.

These were the applications that provoked the question that left me dumbstruck. Applications have been made before to build blocks of flats on those sites, so the ability to shrug off major developments on Metropolitan Open Land can't be discounted once the principle's conceded.

It would also erode the seamless way in which the surrounding areas blend into the Heath, and the Heath in turn extends its character into the communities around it.

Resistance from the community has enabled the edges of the Heath to remain as organic and natural as possible in 21st century London and ensured that the Heath's character continues to extend into Hampstead, Highgate, and the Garden Suburb.

Yet it was only the obstinate refusal to budge on the small things - as well as the large - that has stopped the Heath being eaten away piecemeal. It needs to be a red line for a reason.