Under UK law your employer arguably has a claim to your IP if any of the following are true:
1) They asked you to work on this as part of your employment
2) You worked on this during your employed hours
3) You worked on this on employer provided equipment
If you can prove that this is an undertaking of your own (especially if it pre-dates employment) and you avoid doing any work (not even replying to Git issues) during work hours, and only ever on your own hardware... then your employer has no grounds for claim at all.
1) Is this a traditional breach of IP law, i.e. the open source project infringing on a patent or IP of the employer?
2) Did this predate your employment and conversation to be employed? i.e. could contributing to this project constitute as furthering the employers business interest and reasonably be considered a derivative work from being employed?
The latter seems to be what you're describing, and there are examples in the manual cited above. If it's a different area of work than your employer then it's of no consequence, but yes if you're using your employment to further your understanding and to make something that roughly fulfils the need - then this falls under my earlier comment of "They asked you to work on this as part of your employment" and you would likely have to give up the project.
1) They asked you to work on this as part of your employment
2) You worked on this during your employed hours
3) You worked on this on employer provided equipment
If you can prove that this is an undertaking of your own (especially if it pre-dates employment) and you avoid doing any work (not even replying to Git issues) during work hours, and only ever on your own hardware... then your employer has no grounds for claim at all.