Pulling existing levers may be the most politically palatable way to squeeze millionaires

Pressure to go further on wealth taxes - by creating new modes of clawing at hoards of hard-to-reach cash - is mounting.

- Anna Isaac

For starters, the fiscal picture looks fairly bleak. Economists estimate Rachel Reeves must raise £20bn or even as much as £50bn to meet her goal of balancing day-to-day spending against the revenue raised from taxation.

On the government backbenches, meanwhile, MPs want the chancellor to squeeze the richest harder. They even put forward an early day motion last month calling for a 2% annual tax on individual assets over £10m.

Yet introducing these kinds of taxes is often not straightforward, with the behaviour of the wealthy being hard to monitor and harder still to predict.

The first significant problem is working out where the assets are and who holds them. That has always been difficult and has become even more challenging after one of the most important economic surveys, the household wealth data series, was suspended by the Office for National Statistics because of its low quality.

The upshot is that HMRC simply does not know how many millionaires or billionaires there are in the UK. Without reliable figures, it is extremely hard to write policies, cost them and administer them.