Hi Starjacks
Arrears (or an ex-gratia payment) are disregarded for 52 weeks after you receive them.
If arrears made to compensate for an official error or an error on a point of law are £5,000 or more, they can be disregarded until the end of your award of the means-tested benefit.
ESA Regs, Sch 9, para 11; IS Regs, sch 10, par 7
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
