Grandad's allotment was the allotment of "Grandad" Trotter in Only Fools And Horses. The allotment was mentioned in 2 episodes, "The Russians Are Coming" (13 Oct 1981), and "Mother Nature's Son" (25 December 1992), and was actually seen in the second of those two episodes.
In 1981 in "The Russians Are Coming",Grandad Trotter had an old allotment that he had not used for 19 years since 1962. Derek "Del Boy" Trotter and Rodney Trotter were planning a dummy run in case there was ever a 4 minute warning of Russian missiles attacking Britain and Rodney suggested doing a one so they got their stop watch. In a couple of minutes they managed to get into the van but as they were driving through the back streets, they were stopped by Eric The Policeman. Eric was a friend of Del's and let him off. When Del asks Rodney how they are doing for time Rodney says "we died 45 seconds ago". The allotment was never seen.
11 years later, in late 1992, "Mother Nature's Son" Grandad had been dead for almost 8 years and Del Boy found out that his paperwork had come thorugh on his flat and that the allotment came with the flat. Del also received a court summons to clear up the allotment as people were dumping toxic waste which could be an environmental hazard. That day, Del, Rodney and Albert went to the allotment to dump some of their old junk in the shed. By chance they saw their friend Trigger there. They found several drums of toxic waste there and contacted Denzel Tulser to help them dump it.
Del saw a money making venture when he decided to have a "spring" built so that he could have the water bottled and sold. Del got Denzel and Trigger to build the spring. They dug a hole, laid rocks there and attached a hosepipe to the back to make it look like a real spring. Local health food guru Myles was called and was gullible enough to think it was a real spring. Del even had some Buxton water poured into a child feeding bottle to pretend it was water from the Peckham spring so that it would pass the lab tests.
Soon the Peckham Spring water was being sold all over England and the Trotters were raking in the money. But the water was tap water from the tap in Del's kitchen. Del even took out a loan to buy all the packing equipment. But Denzel and Trigger had got rid of the toxic drums found on Del's allotment. They dumped them in a local reservoir. The Trotters booked a weekend away to Brighton to celebrate their money. The national news reported that the toxic drums had been found and that the whole of Peckham was without water due to contamination. Del even said that this time next week he will be in all the papers.
Del's 2015 autobiography He Who Dares confirmed that after the Peckham Spring water was discontinued due to being contaminated by the toxic waste, Grandad's allotment was destroyed in a small earthquake.