Where cash is tight, and your biggest debtor is HMRC, then you really need to be proactive – burying your head in the sand (i.e. binning those brown envelopes) will not cover you in glory with the taxman. Instead, ask you accountants to speak with the relevant Debt Management team on your behalf.
Where there is a sound reason (e.g. a major client of yours is in financial difficulty) then it is often possible to negotiate a payment plan with HMRC over a period of time. But be warned, this is discretionary, and you will need to be able to show that you have exhausted all other avenues first.
If your case is accepted, you will need to sign an installment payment acceptance letter, which binds you to the agreed terms – and in most cases, if one installment is late or missed, the agreement becomes void and the debt is then immediately due in full again.