This was covered at the Friends of the Horniman AGM on 23rd June. The Horniman is facing very substantial cuts in its support from the Dept of Culture Media and Sport, so they are having to think creatively about ways of raising revenue and reducing costs.
In the former, they are looking at making a nominal charge for school parties, and raising private funding, in the latter reducing operating costs. The closure of the clock entrance has fallen victim to this.
Think its sad but inevitable - we should do all we can to support this great, and for the most part, free institution.