Quatermass 2

As Professor Bernard Quatermass (Brian Donlevy) struggles to gain government support for his Moon colonisation project, his interest becomes focused on reports of hundreds of meteorites landing in Winnerden Flats. Travelling there with Marsh, his colleague (Bryan Forbes), Quatermass finds a huge complex under construction, based on his lunar colony plans. Marsh finds an undamaged meteorite is shaped like a small stone rocket. It then cracks open, releasing a gas, leaving him with an odd V-shaped mark on his face. Black-clad guards from the complex arrive, armed with machine guns and sporting similar V-shaped marks, and take Marsh away, knocking down Quatermass and ordering him to leave.
Trying to discover what happened to Marsh, Quatermass contacts Inspector Lomax (John Longden), who had previously assisted him (see The Quatermass Xperiment). Lomax puts him in touch with Vincent Broadhead (Tom Chatto), a Member of Parliament, who has been trying to uncover the veil of secrecy surrounding Winnerden Flats. Quatermass joins Broadhead on an official tour of the complex, which he is told has been built to manufacture artificial food. Slipping away from the visiting party, Broadhead attempts to get inside one of the large domes that dominate the skyline. Quatermass later finds him dying, covered in a poisonous black slime.
Shot at by guards as he exits, Quatermass rushes to Inspector Lomax, explaining that he believes that the complex is indeed making food but not for human consumption. Its purpose is to provide a suitable living environment for small alien creatures being housed inside the huge domes. Lomax attempts to alert his superiors, but when he meets the Commissioner of Police, he notices that he, too, is sporting the V-shaped mark; the aliens have taken control of the government.
Quatermass and Lomax then turn to journalist Jimmy Hall (Sid James), who is skeptical of their story but asks to visit Winnerden Flats. At the local community centre, they receive a hostile reception from locals employed to do heavy construction and other work at the complex. The mood changes, however, when one of the meteorite-missiles crashes through the building roof, injuring barmaid Sheila (Vera Day). Armed guards arrive and gun down Hall after he telephones the press. The villagers form a mob that marches on the complex. Rushing the gates, Quatermass, Lomax, and the villagers barricade themselves in the pressure control room.
Realising that earth's atmosphere must be poisonous to the aliens, Quatermass sabotages their life support system, pumping oxygen into the large domes. Simultaneously, Quatermass' assistant, Brand (William Franklyn), sacrifices his life by launching a Quatermass rocket at an asteroid believed to be the invasion's staging point. The individual creatures combine their small bodies to create huge 150-foot tall creatures that soon burst from the domes. The rocket destroys the asteroid with a nuclear explosion. Their base gone and now fully exposed to earth's atmosphere, the giant masses of combined creatures collapse and die. The V-shaped marks disappear from those affected, leaving them with no memory of having been under alien control. As they head back to the village, Lomax wonders aloud how he'll make a believable report on all that's happened. More pointedly, Quatermass questions just how final will that report be ...

Question: Vincent Broadhead has been trying to uncover the veil of secrecy surrounding what?
Answer: Winnerden Flats