Locked Sets
A Locked Pair is 2 numbers that occur in 2 squares in the same row, column or 3x3 block. A set could also contain 2, 3, 4, numbers in 2, 3, 4 squares (etc).
R5C1 and R6C1 both contain <18>, which means that they MUST be either <1> and <8>, or <8> and <1>. Either way around means that R4C1 cannot be an <8>. Similarly this applies to every other <1> and <8> in C1 and Block4, which can therefore be removed.
A locked set may contain any number of numbers, for example R6 contains <467> in three squares, and again, no matter which way around the numbers actually go, R6C3 cannot be a <4> or <6> (and is therefore <5>).
Block6 contains <169> in three squares, so all remaining <1>, <6> & <9> can be removed from Block6.
