UC treatment of arrears of ESA

I have a client who is in receipt of UC. She was also in receipt of ESA (IR). UC deducted her ESA from her UC payments. However in May 2020 she received a lump sum as ESA was increased which covered the period from 2018 to May 2020. UC has calculated an overpayment from 2018 as they carried on taking the “old amounts” of ESA into account.
I am looking at UC Sch 10 para 18 and wondering if they should have ignored the arrears paid and calculate the overpayment from May 2020. Para 18(1)(b) says a benefit abolished by Section 33 of the Act which ESA (IR) is, however Para 18 (1)(c) states that only an SS benefit which is not included can be disregarded. So it seems confusing to me and I want to make sure before advising my client.
Thank you in advance for any help you can give.

I cannot see how your client could possibly have been entitled to/had in payment two different means-tested benefits (that is, I-ESA and UC) at the same time. Are you sure it is not contribution-based ESA (C-ESA) otherwise known as ‘new style’ ESA when paid under the UC system that increased? It would also be helpful to know what precisely the increase consisted of - i.e. it is difficult to see how it was that UC could have been unaware of/ignored for two years an increase due to the general rate of C-ESA/new style ESA uprating annually. More likely would have been the client being paid the basic rate of ESA initially and that then being increased to include either the work-related activity component or the support component - either when there was eventually a WCA decision or when the client successfully appealed the WCA decision (and your client then also receiving significant arrears as a result). And whilst either of those scenarios has the potential for creating a UC overpayment, both also have the potential for increasing the UC your client is entitled to…..