Well, yeah, I would. If it was obvious to me that the board had not been touched since then why wouldn't I. But here's the thing, this has never happened to me and I suspect such a scenario has never happened to you either.
Let me turn the question around PM. You're a competent tradesman, confident in the quality of your work, you’ve just fitted a new consumer unit, you've fitted it with all due care and to manufacturers instructions. Why do you feel the need to underwrite your workmanship for just 12 months?
The more time that passes after I have installed a CU, the more likely it is that someone else will have worked on the installation. Anyone working on any installation will most likely open up the CU, and probably disconnect conductors as part of that work. Someone replaces a shower, fits a new light in the loft, and so on. At that point in time, my guarantee theoretically ends for the parts worked on,
but I won't know that.
Should a loose connection result from that other work, and the MCB burn out, an unscrupulous client could, being unable/unwilling to contact the responsible electrician (or perhaps they did it themselves), and faced with the cost of a consumer unit replacement, call me up, pretend no other work has taken place, and insist that I honour my guarantee.
Within 12 months of fitting a CU, I will probably know the installation fairly well, having tested and inspected as part of the CU change, so it is likely i will spot any changes. After several years, not a chance.
As
@James says, terminals can work their way loose over time, again the longer it is since installation, the more likely this is to happen.
But really, there isn't much need for a guarantee of longer than 1 year. Any faults as a result of my work are likely to show themselves in a fairly short space of time.
You are right though, this has never happened to me. I very rarely get call backs, and it's always due to faulty equipment or user error.