Spot on Paul. We did one like that today.
Owen, I would not clean a carpet like that at all, i would tell the customer its too far gone and to put the money into a new carpet.
You can literally spends hours on carpets like this for a 50% visual clean, the stress is not worth it.
Remember your using a cleaning wand and not a magic wand.
Owen some carpets cannot be cleaned so they look like new again, imagine if you have a 15 year old battered car and took it to the valeters, you will pick up a clean and shiney battered 15 year old car but it wont be show room condition. Just like that carpet wont look like near new once cleaned