This usually means that while the parcel may have arrived at Langley, the Royal Mail hasn't scanned it into their system yet. 18 days does indeed seem like a while for it to be sitting there; is there anything in the package that might have problems clearing customs? It might be worth contacting the Royal Mail to see if they can shed any light on what's going on.

RM will only speak to the sender. I will file a dispute with Ali Express, and hopefully the sender will investigate.

Quote from europa on October 5, 2015, 07:41
Seller replied: Since the National Day holiday so we will delay shipping you will receive the package. I've given up I've shipped to you...Recently, we put the country in the National Day (holiday), so the freight will be delayed. Please cancel disputes okay? I've shipped to you you will receive the package when you do not have to worry".

He then gave me the same tracking number as before (which indicates the item was shipped on 8th Sep.). Does this mean the seller didn't actually ship the item (on the 8th) as indicated?

As Royal Mail does not scan inbound parcel, you can not know the final delivery status of the parcel (either if it is lost or delivered).

However, China Post database (http://track-chinapost.com/track_chinapost.php) shows the parcel arrived UK (Langley) on Sep 13. Normally you should get the parcel by Sep 30. If not, it means should be either lost, delayed by customs or delivered to other people (which means seller gave you wrong number). It has nothing to do with China National Day Holiday.

I don't think you should cancel the dispute. You can wait one more week, if parcel still does not arrive, you should escalate the dispute.

When i try to escalate on aliexpress, i get this message: "Due to the seller has promised you will receive your order before05. Nov 2015. You can not escalate this dispute, please be patient or contact the seller to confirm the delivery status."

So i have had to 'modify' the dispute, which gives the seller another four days to respond.

If he 'rejects' again, it looks like i will have to wait till 5th November. I would order the item again (from someone else), but i don't want to risk ending up with two items!

Anything else i can do?

Quote from europa on October 27, 2015, 08:52
The package has still not arrived, and seller continues to reject the refund. Furthermore, tracking now says:
RI600202446CN is invalid Number! China Post has not received this parcel.

Any suggestions?

RI600202446CN was returned to China from UK. you can not get it anymore. Open dispute to seller and claim refund.