Sorry, it was Dr Dolittle, not Bedknobs and Broomsticks:

"Offended by the construction of a concrete dam built for a film production of Doctor Dolittle at Castle Coombe, Wiltshire, Fiennes and an SAS comrade demolished the dam (using explosives Fiennes had obtained for authorised demolitions, but which by dint of efficiency he had been able to save). Both fled, and Fiennes (who had recently completed a training course on evading dogs) escaped capture - but his comrade did not, and both were subsequently discharged from the SAS and returned to their regiments."