There is no such thing as an EICR qualified electrician. You can just produce the invoices as far as I am aware.
Not quite true, there are two City & Guilds qualifications think 2391 and 2392 the latter was a cut down version, however there is no requirement to hold these qualifications, if you have no written qualifications I see nothing to say you need them to do remedial work, however to inspect and test you are using your professional expertise, and to charge for a written report without having some thing to show you have the skill I think would be seen as fraud.
If a professional inspects another professionals work, then he needs to show he is equal or higher qualified than the one who did the work. It really does not matter what professional, so if I do some work, and I am trained to level 5, and you want to show in court what I did was wrong, then you need some one trained to level 5 or higher to say I was wrong.
So if Joe did some electrical work even if 3 years ago, and he was trained to level 3 and held his C&G 2391 and your inspector does an EICR and says this work was dangerous or potentially dangerous, clearly you have every right to go to Joe and say hey this work is dangerous you should fix it free of charge, if he argues then you have every right to take him to court, so your inspector must also have level 3 and hold a C&G 2391 so there is a fair debate, it would be better of course if the inspector was level 5.
Likely in real terms if the inspector finds work sub standard and Joe will not fix it, then you would go to his scheme provider, who would likely send their guy with a line of letters behind his name to further inspect the work, but you would likely need to produce the inspectors report and if that report was clearly not professionally done, it would be hard to get some one for scheme provider to inspect. We have seen on these pages many examples of inspectors going beyond their remit, and giving a code C2 to items which are not even an electrical fault, for example lack of smoke alarms, which could well be highlighted on a fire prevention report, but if not there clearly not an electrical danger.
But to do the repairs is a completely different situation, missing blanks on a consumer unit do not need any electrical test after being replaced, so a receipt for the blanks, and likely a picture to show fitted, stapled to the EICR is all that is required. Even if the inspector would have failed those blanks as they could be removed without a tool, he is not required to return, so until next EICR your OK. And since the push in blanks are manufactured it is hard to say code 2 likely would only be code 3.