What happens if my company pays a dividend when there are not enough distributable profits?

I run a small limited company in London and I am concerned about a dividend we may have paid without checking the position carefully enough. What happens if a company pays a dividend when there are not enough distributable profits? Is it an unlawful dividend, and does the shareholder have to repay it? How do directors check reserves before declaring a dividend, and what accounts or paperwork should be reviewed? Does it matter whether the shareholder knew there were insufficient profits, and what are the tax consequences if the payment is treated differently? I would also like to know how to put things right, whether a later profitable period can cure the problem, and what records Companies House or HMRC might expect. Practical steps would be very helpful.