I did not see any tour guides about when we visited - they were actually preparing for a service in the cathedral - you would probably be best going to the main castle entrance int he centre of the village(where you pay to get in the older part of the castle) - i think it opened at about 10am - and asking at the desk there where they speak very good English, and there are tour guides about all the time - I am sure one of the guides there could arrange it for you or even take your group over for a small fee - its only a very short walk between the two places - I think waiting at the cathedral for someone to turn up on the off chance would be a mistake