For future info, I just fitted a new timing chain tensioner to cure this problem which was most evident when the engine was warmed up and left for ten mins. A terrible rattle on idle or sitting in traffic, eminating from the front of the engine. I tried the Internet when trying to diagnose this, hence this thread, which someone may find useful one day. It's a relief to report the job is no biggie and cost £230 incl VAT done by Honda.