You should prepare and submit your return as soon as possible to avoid any further penalties from HMRC.
Penalties for late submission are as follows:
Up to 3 months late £100
3 months late £10 daily penalty for up to 90 days (max £900)
6 months late 5% of tax outstanding at that date*
12 months late £300 or £300 if greater*
*the total of the 6 and 12 month penalties cannot exceed 100% of the tax due