In September 1944, Duncan Sandys ordered the constitution of a Technical Inter-Services Mission under Colonel T.R.B. Sanders.
It was given the task of investigating the V-weapons sites at Mimoyecques, Siracourt, Watten, and Wizernes, collectively known to the Allies as the "Heavy Crossbow" sites.
Sanders' report was submitted to the War Cabinet on 19 March 1945.

Even at this stage the true purpose of the site was unclear.
Claims that it had been intended to be used for "electro-magnetic projectors" (railguns), firing huge shells at London,  were debunked by Lord Cherwell, Winston Churchill's scientific adviser, who calculated that it would take sixty times the output of Battersea Power Station to fire a one-ton shell.
Sanders' investigation brought to light the V-3 project for the first time, to the alarm of the British government.
He concluded that although the site had been damaged it "could be completed or adapted for offensive action against this country at some future date, and [its] destruction is a matter of importance."
Sandys brought the matter to the attention of Churchill and advised: "Since this installation constitutes a potential threat to London, it would be wise to ensure that it is demolished whilst our forces are still in France."
Churchill later commented that the V-3 installation at Mimoyecques "might well have launched the most devastating attack of all on London."

The discovery of the site's true purpose produced some recriminations in London, as – unlike the V-1 and V-2 projects – the V-3 had not been uncovered by Allied intelligence  before the war's end.
The British scientist and military intelligence expert Reginald Victor Jones later commented that "[intelligence] techniques that had been used against the flying bomb and the rocket appeared to have failed against HDP [V-3], and there had to be a reason.
Basically it was that with our limited effort we had to concentrate on the most urgent problem, and thus on catching weapons not so much at the research stage (although we sometimes achieved this) as in the development stage – which usually meant when trials were showing promise."
He concluded at the time, in April 1945, that the intelligence failure had not made much practical difference given the fact that the Germans had failed to develop the HDP into an effective weapon: "there was little warning; [but] there was little danger."

Following the recommendation that the site should be destroyed, the Royal Engineers stacked ten tons of British 500 lb (230 kg) bombs and captured German plastic explosive in the tunnels at Mimoyecques and detonated them on 9 May. This failed to achieve the desired effect, and on 14 May, a further 25 tons of explosives were used to bring down the north and south entrances to the railway tunnel into the site.
A subsequent investigation by the British Bombing Research Mission concluded that the entrances had been heavily blocked and that it would be a very difficult and lengthy engineering task to reinstate them.
The British action was taken without informing the French beforehand and infuriated Charles de Gaulle, who considered it a violation of France's national sovereignty.
In September 1944, Duncan Sandys ordered the constitution of a Technical Inter-Services Mission under Colonel T.R.B. Sanders. It was given the task of investigating the V-weapons sites at Mimoyecques, Siracourt, Watten, and Wizernes, collectively known to the Allies as the "Heavy Crossbow" sites. Sanders' report was submitted to the War Cabinet on 19 March 1945.
Even at this stage the true purpose of the site was unclear. Claims that it had been intended to be used for "electro-magnetic projectors" (railguns), firing huge shells at London,  were debunked by Lord Cherwell, Winston Churchill's scientific adviser, who calculated that it would take sixty times the output of Battersea Power Station to fire a one-ton shell. Sanders' investigation brought to light the V-3 project for the first time, to the alarm of the British government. He concluded that although the site had been damaged it "could be completed or adapted for offensive action against this country at some future date, and [its] destruction is a matter of importance." Sandys brought the matter to the attention of Churchill and advised: "Since this installation constitutes a potential threat to London, it would be wise to ensure that it is demolished whilst our forces are still in France." Churchill later commented that the V-3 installation at Mimoyecques "might well have launched the most devastating attack of all on London."
The discovery of the site's true purpose produced some recriminations in London, as – unlike the V-1 and V-2 projects – the V-3 had not been uncovered by Allied intelligence  before the war's end. The British scientist and military intelligence expert Reginald Victor Jones later commented that "[intelligence] techniques that had been used against the flying bomb and the rocket appeared to have failed against HDP [V-3], and there had to be a reason. Basically it was that with our limited effort we had to concentrate on the most urgent problem, and thus on catching weapons not so much at the research stage (although we sometimes achieved this) as in the development stage – which usually meant when trials were showing promise." He concluded at the time, in April 1945, that the intelligence failure had not made much practical difference given the fact that the Germans had failed to develop the HDP into an effective weapon: "there was little warning; [but] there was little danger."
Following the recommendation that the site should be destroyed, the Royal Engineers stacked ten tons of British 500 lb (230 kg) bombs and captured German plastic explosive in the tunnels at Mimoyecques and detonated them on 9 May. This failed to achieve the desired effect, and on 14 May, a further 25 tons of explosives were used to bring down the north and south entrances to the railway tunnel into the site. A subsequent investigation by the British Bombing Research Mission concluded that the entrances had been heavily blocked and that it would be a very difficult and lengthy engineering task to reinstate them. The British action was taken without informing the French beforehand and infuriated Charles de Gaulle, who considered it a violation of France's national sovereignty.
Fortress of Mimoyecques