Overpayment caused by arrears of ESA

The scene: Client succesfully appeals against WCA, placed in WRAG after a year. Gets £1.8k arrears of contributory ESA. As client’s Applicable amount is £65.45, the arrears create an HB o/p of £900. HB guidance says this is correct approach, to treat the revised award as starting from the due date, not the actual date of the decision.
The law: My reading of HB regs 2006, reg 79 differs, specifically para (6) ‘Where the change of circumstances is that income…..other than a benefit or an increase in the amount of a benefit under the (social security administration) Act is paid in respect of a past period, the change of circumstances shall take effect from the first day on which such income ....would have fallen…’ 
My reading of this is that this is only for INCOME OTHER THAN BENEFITS, and that contibutory ESA is a benefit, but isn’t understandably specifically named in the SSAA 92 or the Contributions and Benefits Act as one of those benefits. Unhelpfully, ‘benefits’ is not defined in the definitions and citations at bengining of HB Act

Q1: Is contributory ESA a ‘benefit’ for the purposes of HB?
Q2: If so, is HB office right or wrong to create o/p?
Q3. If O/P is correct, what powers is HB office using to recover o/p, as it is not client’s fault and no misrepresentation has occurred?
Q4. Am I merely being daft in failing to understand and accept the inevitable droite de segneur of HB?

Maybe i’m missing something here but surely his HB applicable amount will be increased by the work related activity component as well, meaing no overpayment?

There ought to be no overpayment because HB Regulation 22 provides:

Applicable amounts
22. Subject to regulations 23, 24, 80 and 81 and Schedule A1(p) (polygamous marriages, patients, calculation of weekly amounts, rent free periods and treatment of claims for housing benefit by refugees), a claimant’s weekly applicable amount shall
be the aggregate of such of the following amounts as may apply in his case

(a) an amount in respect of himself or, if he is a member of a couple, an amount in respect of both of them, determined in accordance with paragraph 1(1), (2) or (3), as the case may be, of Schedule 3;.........

(e) the amount of either the–
(i) work-related activity component; or
(ii)  support component,
which may be applicable to him in accordance with Part 5 of Schedule 3 (the components).

Thank you both. HB made an error in only noting the change in income but not applicable amount. Oddly, the guidance they used to justify their original decision ( an example of a succesful ESA appeal creating a recoverable o/p) is wrong. There is now no o/p. Thanks again.