This girl hatched 5 weeks ago (two weeks old in picture), The egg from which she hatched came from a RIR, but she looks like a welsummer?
She's noticeably different to all her sisters yet they all came from the same batch.... Any Ideas?
That is what I call a "wild" gene pattern. Some lesser strains of Rhode Island Reds do indeed throw chicks in that pattern.
I see you are in the UK. The lines of Reds in the UK are somewhat limited, both in quality and in number. You'll have to wait and see how these chicks turn out.