By Alys Denby When one of the most senior figures in investment banking was asked at a private event last week what keeps him awake at night, he replied “government debt”. A recent House of Lords report will not help him sleep any easier. The Economics Affairs Committee has warned of the “grim reality” that at £2.74 trillion – 99.4 per cent of GDP – public sector net debt is close to becoming unsustainable. Without urgent changes to fiscal policy, the UK could face a market reaction to make Liz Truss blush. Forget the £22bn ‘black hole’ the government believes the Tories left behind, this is t…