You can be sure that £6000 was monthly guys = £72000 yearly turnover. Now it should be less, working on his own. But prob. still £2500 or so a month.
Call up accountant, get it sorted. If you don't have money for tax, then you haven't set aside 27% every month of what you earn. And that's not smart.
Even better, do it yourself. You need to register first for online assesment, you get the account number and password in the post. Do it now. Sitting down doesn't help, same with drinking beer. Get moving, right now.