I’ve had a few issues with my erider 15. What I’ve found is that your local bike shop or mobile bike mechanic should be competent to do the mechanical stuff such as you are describing. Most charge by the hour so ask about their rates. For anything involving the electrics you should go to someone who specialises in electric bikes – but they probably charge a higher rate. I live near Swindon and have found eMotion in Gorse Hill to be competent. Suggest you ask a cyclist about local bike repairers and maybe Google for electric bike specialists.
Good luck. Tim Lulham.