The chief exec wasn't a failure: it's worse than that, she was replaced because she had been appointed by the previous administration and Labour wanted someone more in tune with what it was doing. That's not an anti-Labour point: the Greens did exactly the same thing, as did the Tories before...
From the government website
"The Financial Secretary is responsible for:
deputising for the Chancellor at Ecofin
EU Budget and wider EU issues
strategic oversight of the UK tax system including direct, indirect, business, property and personal taxation
corporate and small business taxation...