A little clarification is needed here. An ECS card is not a certificate as an electrician, it is the electricians equivalent of the CSCS card which is a health and safety certificate, nothing more. JIB will not give you full accreditation without a level 3 certificate (2330/2365/2357) and an NVQ level 3. NICEC will accept anyone with three pieces of paper, 17th edition, either 2391/2394/2395 and a cheque.
Since you already have a 17th certificate, to join NICEIC you just need a level 3 certificate in Inspection and testing Last time I looked it cost ÂŁ576 for a new start up fee plus ÂŁ350 for a site assessment You are looking at over ÂŁ900 and then you have to buy their encyclopaedia on their take on the wiring regs for another ÂŁ400-ÂŁ500. It would be cheaper and better to do 2394 and the AM2 which is an industry recognised trade test
Your easiest option would be to do the NVQ3, you should have no problem if you are working on site as a mate. I hope this is helpful