Whilst the banks have done a lot wrong (and heads must surely roll), I find the expectation of certainty that the opposition expect ridiculous.
The reason this recession/depression is so bad is because of its speed and scale, we haven't seen anything like this before.
Therefore if we ignore why we got into this situation for just a minute (and as I said, heads need to roll for this), if anyone does claim to know what lies ahead they are, frankly, talking through their holes. Nobody really knows how much money will be required. You can only react to what you know and can reasonably forsee.