When the continuing surviving adult dependant pension is first put into payment, and it is less than £260.00 a year* it will automatically be converted to a one off payment (known as trivial commutation).
*A surviving adult dependant pension cannot be commuted, even if the pension is below the commutation limit, if there is at least one dependant child eligible for a children's pension at the time of death.
Note: It doesn't matter how small a children's pension is, it cannot be commuted.
Where the continuing surviving adult dependant pension is £260 a year or more, the pension is automatically authorised as a monthly pension. Although spouses/civil partners/nominated partners are currently not given the option to commute such small pensions into a lump sum when they first claim, NHS Pensions will consider a written request at the point of claim or post payment, provided all conditions are met.
Important note: It is possible to convert a small surviving adult dependant pension to a one off payment, however His Majesty's Revenue & Customs (HMRC) only allow this if the gross trivial commutation amount does not exceed the current commutation limit of £30,000.00** (£18,000.00 for a lump sum paid before 6 April 2015).
**This is the maximum amount in each NHS Scheme, not the maximum across all pension schemes.  The £30,000 is gross before taxation.
The recipient must agree to offset any pension already paid against any trivial commutation lump sum. 
Note: HMRC can change this limit at any time.