The turret clock is a small edition of the Great Westminster Clock (Big Ben) and was erected by the parishioners to commemorate the Diamond Jubilee (60 years) of Queen Victoria's reign on 20 June 1897. When the Queen came to inspect the clock on 15 July 1897 she was late and the Rector, the Revd Arthur Robins, asked that the hands to be moved back so as not to embarrass the Queen but when she arrived her first words were "I see, Mr Robins, that your clock is behind time".
The clock strikes every hour and marks the quarter hours from 11am to 7pm with Westminster chimes. The clock is usually stopped for a day when we move in and out of British Summer Time. It is also usually stopped for our Remembrance Sunday Service.
Unfortunately the clock is very sensitive to temperature changes and gets excessively slow when the temperature is very high (so we stop and start the clock at the right time when the weather cools).
Update 23 March 2020: The clock is working to time after being stopped for three weeks due to work on the spire and inside the tower (the bells are very loud if you're beside them when they're struck!). The clock will be stopped again at the end of the week and restarted at the correct time the following week as we move into British Summer Time. The spire requires some structural work and if you look up when you're next passing you might see the net which now covers it.