Three men were pouring concrete to fill in an opening in the first floor of the store, left following the removal of stairs and an escalator.
The Old Bailey heard that on 21 September 2006 the men had only just begun to pour the concrete when the temporary propping erected to support the underside of the decking, installed within the opening, collapsed.
This caused the men to fall more than 4.8 metres to the floor below.
All three men were taken to hospital, one with a dislocated shoulder, another had a fractured pelvis and elbow, the third worker was unhurt.
Bridgford Interiors, of Lichfield, Staffordshire pleaded guilty to safety breaches and was fined £25,000 and ordered to pay costs of £23,392.
HSE Inspector Viv Neaverson said: “This prosecution arises as a result of Bridgford’s failing in their duty as the company responsible for the temporary work.
“Whilst they had appointed a structural engineering company to execute the permanent works, they had not appointed a temporary works engineer and, as such, this duty fell to them.”