When UK chancellor George Osborne told the country's public-sector workers last week that they will have to pay more towards their pensions, he promised he had "no plans" to ask for more than an extra 3% of salary, on average. But some people worry that his numbers don't add up.

The public-sector pension schemes are complex beasts with many moving parts. But getting a handle on their finances is quite crucial for the UK's public accounts, as the total bill adds up to just about £1 trillion, on the government's figures.