This publication explores the accuracy of advanced learner language of students of English philology, and the possibilities of enhancing it. It aims to verify the efficacy of targeted educational intervention aimed at raising awarenness of errors typically committed by Czech and Slovak learners of English, and at limiting the number of errors. Quasi-experimental methodology adopting pre-test and post-test was employed, using a convenience sample of 70 undergraduate university students of English. Several research tools were applied: grammaticality judgement test and certainty-based marking, corpus analysis of samples of free spoken and written learner production, and questionnaires providing contextual information. Descriptive statistical analysis of the results manifested the statistically significant positive impact of targeted intervention in the pilot group in both explored aspects, the ability to identify and correct errors and certainty of answering. The experimental group has improved in three of the four aspects under scrutiny, in identifying erroneous sentences and providing correct answers, in certainty of answering and in the accuracy of spoken language. This improvement was statistically significant. One of the major contributions of this research is identifying major error-prone areas of advanced learner English of Czech and Slovak learners. These are in particular grammar errors, especially in the use of articles and lexical errors, most likely committed as a result of negative L1 transfer. An annotated learner corpus which was devised as an analytical tool for this project can be used in future research to examine other aspects of advanced learner language.