The downstream deterioration in macroinvertebrate scores and in levels of key determinands at both low and high flow on Pymmes's Brook over a period of 7 years can be related to the pattern of SSO entry and SSO catchment (track‐back surveys). Although the macroinvertebrate scores and other data support the NRA designated 2B class in the headwaters, sudden quality deterioration occurs downstream which in this paper is related to problems of foulwater contamination at one particular SSO during the surveyed period. Escherichia Coli surveys in the middle part of the basin are shown to be primarily related to this foulwater overflow, and the situation is shown to have slightly deteriorated since 1985. Data collected during sample storms suggest, however, that the “first flush” phenomena is responsible for a wide temporal variability in determinand concentration as well as E. Coli count, the latter increasing even further during storms. Proposals to improve the situation via provision of an East Barnet low level foul sewer (London Borough of Barnet) are evaluated. The implications of these results for the NRA sampling and classification strategies are finally considered.