Theres three scenarios here.
One is you have a piece to repair thats in carry on luggage
Two is it is in the hold
Three is you are importing via post.
One and two no dramas, go to the queue thats the longest, thats the something to declare queue.
You be polite and say Ive got this dooverlacky with me and I want to get it fixed here by an expert.
Customs AKA BORDER FORCE will say oooh thats nice, what is it, give it the once over for drugs and wish you a nice day.
If you mail the item, just declare its for repair, no duties no charges. FREE TRADE AGREEMENT IS IN FORCE