Wilhemina- ok- the issue is the difference between a loan and an equity investment.
£400bn was not lent to the banks (although this is often cited as fact by the media so it's a common misconception).
£400bn was injected into the banks as an equity investment- i.e. exchanged for shares in the bank, so that the government/ nation is the majority shareholder (ie owns most of) RBS and Lloyds/HBOS/ whatever they're called now.
Therefore there is no interest payable (because that's not how shares work), but the government owns the shares which are publicly listed and have gone up in value since they were bought. Have to admit I've lost track but my understanding is they are now almost back up to break even- i.e. the country has it's £400bn back. However, obviously the issue is that the government doesnt have the cash because they'd have to sell the shares to get that. Again, my understanding is that at some stage they plan to do a de-nationalisation and sell them off to institutional and retail investors.
Now the government has a problem. They could say "RBS and Lloyds arent paying bonuses for the reason that we had to bail them out etc etc" which would be completely legitimate. However, they cant stop all the other banks from paying bonuses, other than urging restraint. That would create a problem of competitiveness whereby good bankers wouldn't want to work for RBS and Lloyds, so those banks would become worse instead of better, jeopardising the tax payers shares in those banks.
To look on the bright side, if these bonuses are paid, 50% goes straight back to the coffers as income tax. If the banks keep the money, it's only subject to corporation tax at 30%, so the tax haul is actually higher if the bonuses get paid.
So personally, I think they made the right choice, although I accept that it's wrong on a number of levels.