This requirement in the Scope is very general, but is still a requirement all the same. In terms of liability, it depends upon what design obligations you have under the contract and who has been appointed as the 'principal designer' under the CDM Regulations.
It looks as though there are 2 main issues here;
- CDM compliance,
- contract change management.
The 2nd one frequently gets 'lost' in the design process and is often treated as 'design development' when changes are made. To overcome this I suggest creating a design requirements document which baselines the scope of works. This can then be updated as the design develops, with any changes to requirements dealt with appropriately under the contract, where necessary, taking account of genuine 'design development' issues.