Yes, Himalayan Balsam. Pull it up BEFORE IT SEEDS, put it in a bin bag with some water, let it rot down and take it to the tip. The seeds are catapulted high and for yards and yards, so it's likely that one of your neighbours has a plant.
At our last address a next door neighboour sowed a few seeds. The flowers looked lovely and the bees loved it, he said. The following year we had seedlings springing up all over the place. They're pretty easy to identify even when they're quite small so we just made sure we pulled everyone we saw up and destroyed it. If you pull it and leave it on the soil it will re-root!
Within 3 years the next door garden was totally overgrown with HB, and the tenant moved out. The landlord mowed it down and installed a membrane and shingle. I bet seedlings are still trying to find a way through or around the membrane.
We moved - not because of the HB, but because of the unpredictability of having ever-changing tenants next door - and to get a much bigger garden