Locksmith mill hill prices seems to be decent, I've change a lock on my patio door, its an UPVC and it took me nearly 50£ so I think the price is good, regarding silicon spray, well to be honest it's much better than using WD-40 but it does not prevent you from a broken lock or a stuck key in the lock. You have to spray the locks for a better work but it does not mean you won't have to change a lock in a couple of time