Nurseries are different in the way they deal with injuries and illneses but I think head injuries should be quickly reported to the parent. If the nursery is policy is clear, then staff should not be worried about 'upsetting' parents who feel they are too busy to hear about an accident.
Forget reporting anything to Ofsted. They are completely useless. They do not 'adjudicate' regarding the complaint (i.e. they cannot say who is right and wrong). They simply consider whether the nrusery, at the time of their visit, is complying with national standards.
I have been involved in two complaints to Ofsted - both about ratios. I once witnessed only two members of staff looking after 20 children at a nursery. Later at the same nursery, I found two children, unsupervised and unmissed in the reception area which has open access to the road.
Ofsted investigated. They found contraventions of the national standards. The owner was not even keeping records confirming the staff on and had not CRB's staff.
However, within TWO months, they allowed the nursery to expand to take on more children.
You will cause yourself alot of stress and get no joy. They are too worried about stepping on business toes.
Write in straightforward and amicable terms to the manager, confirming your concern and suggestion a policy be established for reporting such incidents.