yourlocalcat in post #3 had the correct idea
in the current GCSE spec, students are expected to know more sequence types than just linear ones.
linear sequences are still common, but there's now usually a quad sequence question on one of the papers, too.
other sequence types to be covered are:
Fibonacci
& other simple term to term
nth term sequences:
simple geometric sequences, involving powers of 2 or 3 (perhaps also 1/2 or 1/3)
hth