TC overpayment while off sick

Opinions required on this scenario

Client lone parent working 16 hpw nat min wage.

gets £75 a week WTC

Became too ill to work in April, and as below LEL she couldnt get SSP

She rang to claim ESA, gave her circumstances, but was told they wouldnt take the claim, as her 75 quid a week working tax credits would wipe out any entitlement to ESA (wrong, but DWP arent always known for being right)

5 months later Tax credits say she wasnt entitled to any WTC for this period, as no SSP in payment and no ESA claim existed.

Employer has left her job open, but shes been living on Tax credits all this time, and they want it back.

Although grossly unfair, can tax credits actually recover?

Thanks

Claim ESA credit only now with a 3 month backdated sick-line, should help part of any alleged overpayment.

page 957 cpag : credits for LCFW

If i manage to get NI credits for the period then client still treat as being in work, therefore tc overpayment written off?

or have i got it wrong?