Who can audit?

I am the FD of a small UK company (meeting the Co House requirements); we are 100% owned by an EU company and are part of what would be (under UK legislation) a medium sized Group.

My questions are:

1. Are we required to have a audit? (I believe yes as we are part of a medium Group?)

2. We are already audited by our EU Parent Co's auditor who complete the audit of the Group and prepare the final individual company and Group consolidated statements; would this audit 'count' or do we require a separate audit by a UK registered auditor?

Thanks for any help or guidance in advance.