According to this Guardian article Peter Watt accepted the Abrahams donations on the basis of legal advice. The article actually says "the arrangement was technically legal". There's no such thing as "technically legal" - what that means is "legal".
Not a fashionable thing to say then, but if we were able to find lawyers in the past who thought it was legal, we would be able to find barristers who could make that case in court, and rather than him resigning and us all running round screaming mea culpa we should have stood by our original judgement and defended it through the courts. Who knows, we might actually have won.