UDA allocation is based on the completion date of treatment; for incomplete treatment UDA allocation is based on the date of last visit.
If treatment was started March 1st but completed April 1st the claim would be awarded to the new financial year, as the date of completion decides the UDA allocation.
A Course of Treatment (CoT) must be completed on or before March 31st to be allocated to the financial year ending March 31st.
The number of UOA awarded is determined by the patient's Date of Birth at the date of assessment.
UOA for 'Assess & appliance fitted' CoT are allocated to the financial year on the 'Date Appliance Fitted'
UOA for 'Assess & review' / 'Assess & refuse' CoT are allocated to the financial year on the 'Date of Assessment'.
More information can be found on the NHS Dental Services website. The Guidance for 2014/2015 Year-End Statement of Activity - Final Version (PDF: 168KB) covers this scenario, although the guidance is dated 2014/2015 the same rules apply.