Mick E.
Food for thought for you:
You mention a Spur to the garage – does that spur run in series or parallel to your kitchen ring main that you described. (It does matter).
Most Sparkies would put a garage on a separate ring main as a matter of course, ideally with its own consumer unit. This seems like an afterthought and a parallel spur, which is OK until you start pulling bigger power.
You mentioned melting –a scorched socket is normally just replaced – the wiring is normally ok after overheating a little. (you check)
People have mentioned loose connections and they are bang on; planned preventative maintenance in places I have worked include things like thermal cameras (such as Fire brigade use) to inspect High Voltage connections monthly, to detect these issues.
Wherever two cables connect, they create heat, if the thing connecting them is loose, that heat increases.
Worst case scenario based in what I have read above in this post – you have a parallel spur to the garage from the kitchen ring. You are plugging an extension lead into that spur, to reach the car charger?
It’s impossible for anyone here to remote diagnose the problem - If you’re not comfortable with anything above, and you charge regularly, I’d suggest you get a Sparky in. Either to create a separate ring for you Granny charger / garage or advise you on a Domestic EV charger.
All the best