Need repair on toilet flush valve and refill mechanism

Hi there, I have a toilet with a faulty flush and inconsistent refill. The toilet seems to run occasionally after flushing and the tank doesn’t fill properly. I’d like a trusted handyman to diagnose and fix the issue, or replace a worn-out fill valve if needed. Preference for someone who can bring compatible parts and explain the problem in simple terms. Located in the UK; please include an approximate time estimate and any parts cost you anticipate. Access is straightforward, and I can be flexible with the start time.