You should explain your circumstances to HMRC and offer to pay by instalments. They are likely to accept the instalment provided it is reasonable in view of what you owe. Be aware that the HMRC charge interest at a daily rate.
If you do not offer to pay by instalments then the consequences will become more serious which may involve legal proceedings, including a summons to Magistrates Court - it is the last thing you need.
If you do decide to make an offer to pay by instalments, put it in writing and if you can enclose the first payment - this will be more persuasive. You should also send by recorded delivery and keep copies of everything.
If you have an Accountant, it may be a good idea to get him/her on board to confirm your situation.
