If you are putting someone's else name in the declaration at the end of the F10 (HSE electronic submission)as they are the nominated company competent person an act of Forgery?

You are, surely, asking the wrong people here. Ask a real lawyer not the barrack room ones, or perhaps ask HSE since it is their form

IB has a good point In my view; Provided the person has given U permission to sign on their behalf and they know of their duties etc I would say that it is OK The problem we have is when CDMC's do not get involved with the client directly as there are to many middlemen and similar situations in between and therefore a client, should there be a problem, will have a 'get out' if the CDMC has not been in contact and step back from the situation and cover their back

Corbett The Client has to appoint A Designer, A CDM Coordinator and a Principal Contractor. The appointments must be in writing, either e-mail or letter. No written appointment then the client assumes the role. It is also considered good advice for the client to write a letter to indicate they have taken on these roles themselves if they wish to do so. As said previously by putting some ones name on the F10 then they must have been appointed (and accepted) they must be competent to carry out that role.