edam - in practice, Ofsted will do very little save check the nursery's paperwork.
The key is that they have no power to act as judge/jury on a complaint by a parent. So, although they will accept complaints, they will not investigate the complaint itself but rather will use it when considering whether national standards are being complied with.
I took this up at a very senior level as the whole system is very misleading to parents who believe they can complain about poor practice and get something done about it.
Ofsted seem to lack the statutory powers and the will to do anything.
For example, a few years back, they made a policy decision to list any complaints received on the Ofsted report. Nurseries were up in arms about this and the policy was changed after discussions were held between those who represent nurseries and Ofsted. Without public discussion or explantion but just because nurseries did not like it, they dropped this policy. It simply disappeared from their website.
Now, unless, they find a breach of the national standards, they can and will do nothing.
Example, you might complain that the nrusery is running under staff/child ratio which breaches National Standards. However, if Ofsted attend and are shown a record of the staff there, including peripatetic music teachers, the cleaner and the cook, and the numbers add up. End of. They will not specifically investigate the complaint you make. They will simply say whether the nursery is breaching standards on the day they attend.
Stressful and pointless. You've more chance of getting something done by working with the management.