The SEND Code of Practice 2015 says:
Where there is an EHC needs assessment, it should be an holistic assessment of the child or young person’s education, health and social care needs. EHC needs assessments should be combined with social care assessments under Section 17 of the Children Act 1989 where appropriate. (10.18)
In seeking advice and information, the local authority should consider with professionals what advice they can contribute to ensure that the assessment covers all the relevant education, health and care needs of the child or young person. (9.49)
This means that, as part of an EHC needs assessment, advice from social care about their previous involvement and assessments for your child will be sought.
If their response is ‘not known to service’ – you might want to discuss, or ask social care to carry out an assessment (or re-assessment) of care needs.