Their latest message is "ok well its not arived and its been 7 days plz advise thanx".
wtf advise am I supposed to give. Other than, next time when you need something in a hurry, ask the seller if they will provide a tracked / quicker delivery option.
I have proof of posting so was going to post something like
I have the proof of posting so can send that to you. However Royal Mail do not consider a parcel lost until 15 days after expected delivery (which would have been the 24th / 26th of November) so they will not do anything until the end of next week.
I'm afraid there is nothing we can do but wait and hope it arrives before you need it.
Think that's OK.