How do I source/eliminate the sour, spicey, musty odour in our living room?
Try covering the carpet with a sheet of plastic, and see if the smell gets better above and worse below it. Make sure it's a continuous sheet, not a woven plastic tarp. Use one of a reasonable thickness, as polyethylene (the material almost always used for these) passes vapors surprisingly easily.

Put down the plastic in the evening. The next morning, go in and exercise your sniffer; is the smell in the room better? Then lift a corner of the sheet and sniff the carpet: is it worse?

If so, then you might even be able to localize the smell further by sniffing at various points in the carpet as you roll up the plastic.