You'd better phone the CSA up. It says in their leaflet (in respect of benefits including industrial injuries benefit):
If the non-resident parent is getting any of these benefits, we normally take the child maintenance direct from the benefit or Pension Credit. If the non-resident parent�s partner who they live with is getting income-related benefits or Pension Credit, we take child maintenance from their benefit. So the non-resident parent or their partner will receive less in benefit.
The parent with care can apply to get the child maintenance
increased if the non-resident parent:
� pays the flat rate because they are getting certain benefits, but
� they have other weekly income of more than �100 that we would normally take into account.
I would take that to mean that if you got the industrial injuries benefit and nothing else, you would pay the flat rate of �5 per week.
Because you have earnings as well, you pay more.