Maybe I'm getting my knickers in a twist!
Being a "has been" in the training world (with limited access to manuals) I've been looking at CAP698 ,1999 edition, and CAP698 2006 edition, contaminated take-offs. In the former on a contaminated take-off if V1 was less than VMCG then it would be increased to V1=VMCG. The later CAP prohibits the take-off. I've been trying to establish the reason why hence my original post asking what was stated in current performance manuals. I think the description given in my link on minimum control speed considerations page 20-27 confirms why the later CAP is correct.
RHC- not a real situation thankfully.
BOAC. If I've got it wrong please say so. Happy to IC if you prefer.