I’d push the dealer for replacement FOC however, could it of being pinched by some thieving git when parked up somewhere?
I wouldn’t advise claiming through insurance, it will cost more in excess and be classed as a claim. On another note, its not likely to impact your GCB, if you left as is, but for the sake of £20 if the dealers not prepared to help, a used one would be more than good enough.