The mother should not be getting child benefit as she is no longer responsible for your daughter and she should have notified the Child Benefit Office when your daughter moved in with her grandparents
https://www.gov.uk/report-changes-child-benefit
https://www.gov.uk/child-benefit-child-lives-with-someone-else
Regarding the child support you pay, this should have been paid to the grandparents and not your daughter while she was living with them, unless she was using the money to pay the GPs for her keep.
Both you and her mother should be financially supporting your daughter unless she is receiving benefit such as income support in her own right (or as part of her partner's claim). Certain training courses allow for 17 year olds to claim income support. This should have been the situation whilst she was living with the GPs - both parents supporting her financially and GPs getting the child benefit.
Now she is living with her boyfriend it gets tricky. Child support is supposed to be paid to the primary carer (the resident parent or guardian, in your case the mother then the grandparents) but as she has removed herself from parental care (or loco parentis care) there is nobody to pay the child support to. Technically child support has stopped and you are now giving her voluntary payments towards her living expenses.
You can stop these payments at any time you choose and these payments could affect her entitlement to any sort of benefits she may be entitled to.
If your daughter goes to university that is a whole different ball game you need to be aware of. If she is financially dependent on you then your income will be taken in to account if she applies for a student grant. If she is living with her boyfriend then his income will be taken in to account. This applies until she is 25. There is no guarantee your daughter will be living with her boyfriend for that time.
You haven't mentioned what sort of relationship you have with your daughter, nor your own financial situation. Assuming you are a man of moderate means with a good relationship with your daughter I suggest that you stop the weekly or monthly payments, ending the child support. This will mean she is no longer financially dependent on you in any way.
However, you can give the equivalent amount as gifts either in cash or goods - you will both need to be in agreement. You don't pay her monthly phone bill but you can buy her books for college; you don't contribute towards her rent but do give her x amount at Christmas and birthday; you don't buy her bus pass but do give her the equivalent as a gift so she can buy her own. You buy her the odd pair of shoes or pay the occasional electricity bill. Do you see what I'm getting at?
If you have no relationship with your daughter that is a different matter and you could cease all payments now if that is what you want to do.