But while tidying the garden soon after moving in, he discovered Japanese knotweed canes behind a large St John’s wort bush which was growing next to the shed.
Japanese knotweed is an invasive species, notorious for its propensity to spread and cause damage to building structures, as well as the difficulty and expense of getting rid of it.
Mr Downing sued Mr Henderson over the discovery, accusing him of misrepresenting whether there was knotweed at the property when he sold it.
In a ruling at Central London county court, Judge Jan Luba KC found in Mr Downing’s favour, ordering Mr Henderson to pay £32,000 in damages plus his opponent’s legal bill of up to £95,000.
The court heard that during the sale, Mr Henderson had answered “no” on a form asking if the property had been affected by knotweed. He argued that he “reasonably believed” he was telling the truth when he filled in the form.
Mr Henderson claimed he could not see the knotweed because of the large bush, which also probably stunted the weed’s growth before it shot up when the shrub was cut back after Mr Downing moved in.
But the judge heard evidence that the knotweed had been previously treated with herbicide, and may have once stood at around 2m tall.
Outlining the case, Mr Downing’s barrister Tom Carter an expert said the weed had probably been in the garden since at least 2012, three years before Mr Henderson moved into the property.
When the sale went through in 2018, Mr Henderson “chose to positively assert there was no knotweed at the property and thereby made a misrepresentation”, said Mr Carter.
Mr Downing sued for £32,000 to cover the costs of investigating and excavating the plant, as well as the diminution in value of his home caused by the knotweed incursion.
His barrister said there was no way that Mr Henderson could prove that he had a “reasonable belief” that there was no knotweed present at the time he filled in the seller’s forms.
“The defendant cannot discharge the burden on him of showing that he had reasonable ground to believe that the property was not affected by knotweed,” he said.
Giving evidence, Mr Downing told the judge that - had Mr Henderson said it was “not known” whether the property was affected by knotweed - he would have looked into it further.
But in his own evidence, Mr Henderson said he had no reason to think that there might be knotweed in his garden.
“I had lived there for three years and spent quite a lot of time in the garden and hadn’t seen knotweed,” he said.
“I got a surveyor’s report when I moved in and it didn’t find any knotweed.
“No one identified any knotweed to me and I didn’t see any knotweed.
“The main reason is it was hidden by the bush and quite likely to have been hampered by the bush.”
Judge Luba, in his ruling, said: “Mr Henderson told me on oath that he genuinely did think there wasn’t any Japanese knotweed in his garden. He knew what it looked like and he had not seen any in the three years he had been there. His mother was a keen gardener and she made no report to him of Japanese knotweed.
“No previous owners had mentioned Japanese knotweed to him and none of the neighbours had Japanese knotweed in their gardens.
“Had that evidence stood alone, he would have amply satisfied me of his reasonable belief that there was no Japanese knotweed at his property.”
But he went on to say that Mr Henderson’s case had been undermined by his admission that he “didn’t know what was behind the shed” where the knotweed was lurking.
The judge said his confidence in Mr Henderson’s story was “shaken” further by evidence given by a joint knotweed expert, which suggested that knotweed canes had possibly stood 2m tall at one point and might have been “overhanging the neighbouring garden.”
There was also evidence that the weed had been treated with herbicide at some point in the past, he said.
“The opinion of the single joint expert is that Japanese knotweed growing would have been visible in the garden,” he continued.
“I ask myself whether Mr Henderson genuinely did believe there was no Japanese knotweed affecting the property. I’m not satisfied he has met that burden.
“Even if I am wrong and he did genuinely believe the answer, he has not shown me that he had any reasonable grounds for doing so.
“The defendant is liable to the claimant for the sum of damages agreed.”
Mr Henderson has been ordered to pay the £32,000 damages plus £65,000 of Mr Downing’s costs on account within 21 days. He will also have to shoulder his own legal costs.