Whilst the threat to move Euro clearing from London to Frankfurt might come under the heading of "usual scaremongering", it is realistic - but only if we stay in the EU.
The UK won its case against the ECB's last attempt to make clearing outside of the Eurozone unprofitable, but it did so on a technicality and the court went to great lengths to show the ECB how to fix that technicality. Once the ECB does as the EU Court suggests, it a can prevent the UK from acting a second time to save Euro clearing in London.
The ECB, however, can only use EU law against London if we remain in the EU. Once the UK is free of the shackles of EU law, the ECB's attempt to rule over us will be history.
Then we can continue to clear Euros just as we clear US Dollars.