As you can see it's just a standard power transformer with a changeable voltage giving out 600ma, with a changeable plug that happens to fit the NTSC super nintendo.
From my experience, those power adapters with selectable voltage are usually cheap junk. And it doesn't help that it's not outputting the required power specs.
Would I use a stepdown converter? I'm worried about using them, since I use one currently with my Sega Genesis (same story with NTSC games as I have a Nomad) but, the stepdown converter gets incredibly hot and smelly, and makes noises when its on. So I'm a little worried about going down that route. Granted it was only £6 from eBay but still lol, maybe using a more expensive stepdown converter would be safer?
Well if it was only 6 pounds, it can't be very good. I mean, you said it gets really hot, smells, and makes noises, so we know it isn't very good.
As for the issue at hand, I can see that there is a missing 250mA here, as well as 1 volt compared to what the SNES is expecting to get, so lucky that it does boot with this loss lol.
Right. Like I said in my last post, some of us have used power adapters for various consoles that output less than the required voltagage, more or less of the required amperage, and gotten away with it. Some times though, we're not so lucky. A perfect example is a cheap Chinese 9v adapter I was using with no problem for my Famicom. I tried using it with my model 1 Genesis, and it work, but the audio was heavily distorted. I found myself a better quality power adapter, and the console turned out to work fine.
From all the information you have supplied, here are my suggestions: Get an official (which might be hard) NTSC-U SNES power adapter and use a voltage converter of decent quality. This would be the easiest solution, though probably the more expensive.
My other idea will probably be a lot cheaper for you, but will require a little work and some know-how. The plug that you're using now, it has a tip that fits the NTSC-U SNES. So you got the hardest part out of the way. As long as the tip fits snug and makes proper contact, there's nothing wrong with it. The problem is with the power supply itself, it doesn't output the correct requirements and the quality of it is questionable. Just find yourself ANY power adapter that outputs the required specs, or as close as you can get. I would recommend hitting the voltage dead on, and trying to match the amperage as close as possible. For example, I searched "10v 850ma" on UK eBay and found this...http://www.ebay.co.uk/itm/DVE-DV-10850- ... 53edf55335
I never heard of that brand, DVE, but I also don't live in the UK. Perhaps that's a decent and trusted brand name there. So I'm not suggesting that you buy THIS particular power adapter, I'm just using it for an example.
Also note that 1000mA is 1 amp, so you could try searching for .85a instead.
I searched "10v 900a" on UK eBay and found more results. This one for example...http://www.ebay.co.uk/itm/Akura-AEC1008 ... 46059313bf
Again, Akura, I don't know if that's a decent brand or not. But by now you should get the idea. Don't just limit yourself to eBay, you can hunt thrift shops or whatever local online shops you have by you.
For example, the power supply I ended up using for my model 1 Genesis was a Casio brand. It outputs the correct voltage and about the same amperage, and Casio is a decent brand. Another example was an HP power adapter I got that was for some old scanner.
Anyway, long story short, find an appropriate power adapter. But pay no attention to the tip size/shape. Just make sure it inputs 240v and outputs 10v ~850mA. After you get your new power adapter, assuming the tip wont fit the NTSC-U SNES, cut the tip off of the plug. Then cut the tip off of the selectable voltage adapter you're using now (the one that fits the NTSC-U SNES). Leave yourself a decent length of wire to work with. Splice the tip onto the new power adapter, and make sure you get the polarity right. And that's all there is to it!
But also, use Hobie's suggestion to clean your console good. I'm not saying that the power supply IS definitely your problem with Super Mario, but it's just a possibility. These are just my tips to find yourself a proper power adapter.