Yes, it seems that the concern is "a cluster" that have bern found, in the sewage, regularly, since February.
"Over the past four months, the UKHSA has found the polio virus in samples collected from the Beckton sewage works, which serves a population of four million in north and east London.
Scientists believe the virus originated from someone who was immunised abroad with the live oral polio vaccine, which hasn't been used in the UK since 2004.
That person then shed traces of the virus from their gut which were detected by the sewage sampling.
In rare cases, that form of the virus can then be transmitted to others and mutate into what is known as "vaccine-derived" polio.
Although weaker than the original or "wild" form of the disease, it can still cause serious illness, including paralysis, in people who are unvaccinated.
A tiny number of samples of the polio virus are detected each year in sewage surveillance, but this is the first time that a cluster of genetically-linked samples has been found repeatedly over a period of months."