What do I do if I am unable to submit the tax return for self-employed persons by the deadline?

What do I do if I am unable to submit the tax return for self-employed persons by the deadline?
If it would be very difficult for you to submit your tax return by the deadline, you can apply for deferment of the submission deadline. You may for example be unable to submit your tax return on time because you have been ill. 

The deadline for deferment applications is 31 May.

An authorised accountant or auditor is going to help me complete my tax return. How can I find out whether they have applied for deferment of my submission deadline?
Accountants and auditors can apply for a deferred submission deadline on behalf of their clients. You should contact your accountant/auditor if you are in any doubt as to whether or not they have arranged for your submission deadline to be deferred.