I suppose that the auxiliary battery voltage might be low. Power from the heated seats would be roughly proportional to the square of the voltage, so if the auxiliary battery is down 10% (say 1.2 V lower than normal), then heat output would be reduced about 21%. Is the voltage on the crude voltmeter normal (reading 14 V when in ready mode)?
Otherwise, my guess would be high resistance contact on some connectors, or perhaps the fuse. I have not seen an MG ZS service manual (paper or electronic), so you'd likely have to locate the right fuse using the fuse cover label, if there is one. The seat heater would draw significant current, so if there is a high resistance connector, it would get warm to hot during use. You might be able to spot it by hand or with a non-contact thermometer.
