Hi Bren
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum In answer to your question; arrears or an ex-gratia payment are disregarded for 52 weeks after you get them.
If arrears are 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, para 7; HB Regs, Sch 6, para 9A
Gary