Parts availability may be an issue with JLR vehicles due to the current stop in production, but it is still frustrating that there are no parts in stock. Given that your vehicle is still very new we would expect it to be covered under the original manufacturer's warranty never mind an extended warranty and while it is possible that the DPF may be excluded from warranty terms we would not expect a DPF to fail in under 12 months from new. However, given that the vehicle is so new having it repaired elsewhere would mean paying the whole bill from your own pocket and may invalidate your warranty, so it is likely just a matter of being persistent with the dealer or contacting Land Rover UK in order to escalate the issue.