I've had a look at the clip and the bloke is cleaning out the smokebox and then the ashpan.Cleaning out the Slut and the Slag - The Layout of Locomotive Depot Facilities(b&w/silent)
Locomotive maintenance workers clean out the slut and slag from engines at York Station. (2 minutes 45 seconds ©YFA)
In a thread on uk.railways there are suggestions that '"slag" means the silicious waste left over from metal refining and welding operations. In this case they probably mean "clinker"', while "slut" may refer to "soot" or "sludge".