IB(Y) error and IS overpayment

Client was receiving IS, and doing permitted work. Fluctuations in earnings were not declared, and DWP have claimed an overpayment for periods where they exceeded £20pw.

It seems to us that he should actually have been on IB(Y) throughout, and so benefitted from the higher earnings disregard, which would remove most of the overpayment.

We have appealed on the basis that the claim should have been found to qualify for IB(Y). DWP submit that the issue to be determined at tribunal is a decision on IS entitlement and recoverability, and not the outcome of an IB claim determined several years ago.

Is there any point in pursuing this to tribunal?

We have already asked that the IB decisions be revised on the grounds of official error, but this was sent in the form of an appeal. The original decision is well out of the absolute time limit.

Possibly relevant background, a thread here and a recent case:
http://www.rightsnet.org.uk/forums/viewthread/1872/
http://www.osscsc.gov.uk/Aspx/view.aspx?id=3674

So, I suspect the answer to this question is “No, of course the tribunal can’t deal with an overpayment caused by a faulty decision made several years previously, you need to persuade DWP that they should revise on grounds of official error”?