"I am a little surprised by this, if the claim was untrue why on earth did the conservatives not make a huge fuss about it when it was first stated there was black hole in July 2024, why has it taken 18 months for them to realise."

That black hole was (supposedly) dealt with in the 2024 budget. This claimed 30 billion black hole was a new one (based on Labour policies, not Conservative)
But it seems to have disappeared between Rachel Reeves discussing it as a reason that the whole country might have to tighten their belts, and the budget a few weeks later.