I've accidentally added more yogurt than I should've (I tipped the box of yogurt rather than spoon it out) and now the flavour / smell of yogurt and its sourness are overpowering and coming through. I've cooked another onion down separately and added a tbsp of demerara, a glug of milk (suggestion from my girlfriend) and added it to the sauce (which is in the slowcooker as I type this) but the flavours/smells coming out of it is just too overpowering. Anything I could do to mellow it out?
You can't take the yogurt flavor out, what you can try to do is balance the undesirable flavors and smells out. This may or may not be effective but it's worth a try. You can balance the sourness with sweet and/or bitter flavors. You've added some demerara sugar which is a good idea, for bitterness you could add fenugreek if available. There are comparatively few bitter spices in most people's kitchens, but you will likely have instant coffee and unsweetened cocoa powder, and you may have bitters if you make cocktails. I would shy away from these though as the flavors they would add aren't compatible but YMMV.
If you don't have fenugreek (or you have added some bit it still isn't enough) you could try to add more curry spices like cumin, coriander, and mustard and try to overpower the yogurt taste. How I would do this is to put a small amount of oil in a pan and heat, then cook the spices in it for a short time, say one minute. Then add a couple of ladles of your cooking liquid, and simmer it for a 3-4 minutes before adding it to the dish. This will get the flavors out of the powder and help get them into the dish, if you just add powdered spices to liquid you can sometimes get a powdery taste from them, especially when using a slow cooker.
This may end up making your curry way too strong, even if the flavors are in balances, if that happens remove some of the liquid and replace it with weak stock or water.