NEC ECC: Responding to Compensation Events

We are Contractors on a project in which we have entered into an NEC3 contract with our customer. The customer has received some CEs from another sub-contractor. The customer has decided that these costs are to be borne by us as he is of the opinion that these costs have arisen due to something we requested, and as such has raised a Compensation Event to us.

Two questions please:
(a) can the customer do this via a Compensation Event or should it be done via other means
(b) what form/document/template must I use to reply to them, assuming that the CE is the correct vehicle for this dispute.

Thanks