“ What emerges from all this is a co-operative process of investigation in which both the claimant and the department play their part. The department is the one which knows what questions it needs to ask and what information it needs to have in order to determine whether the conditions of entitlement have been met. The claimant is the one who generally speaking can and must supply that information. But where the information is available to the department rather than the claimant, then the department must take the necessary steps to enable it to be traced ”