If there is a genuine silicosis risk, then the HSE would be able to stop it immediately with a prohibition order. If the sand used either contains fine dust or the blasting process creates fine dust sized particles then I imagine there may be a risk (to the contractor and his employees more than to anyone else).
The threat of a phone call to the HSE may itself be effective, since a prohibition notice will force him to stop until the HSE is satisfied that remedial action is taken. That would mean more than just sheeting, since they would be looking at the PPE and RPE issued to staff, risk assessments, maybe health monitoring etc. etc. In other words, a lot of trouble for the contractor.