I believe you only need a portfolio to get your ACS or what ever the initial qualification is called to be eligible to test for different categories such as CKR1 and the other appliances.
It will depend on your confidence to test for CKR1. I think it has a multiple choice test which you need 80% for followed by a small practical assessment. You could be done in half a day if you don't need any refresher training
If you're already Gas safe I'm almost certain you don't need to touch your portfolio again