After much research we have come to the conclusion that the reference to DME/DME is a bit of a red herring and that what they are really asking is "What is the RNP standard for a non-precision approach?". The following is an extract from ICAO Doc 8168:

Departure procedures are normally based on RNP 1. Where necessary and appropriate, they may be based on RNP 0.5 or RNP 0.3. Departures are not associated with an RNP less than RNP 0.3.

Non-precision approach procedures are normally based on:

RNP 0.5 (initial approach only) or;

RNP 0.3 (initial, intermediate, final approach).

Non-precision approach procedures are not associated with an RNP less than RNP 0.3.

En-route procedures are normally based on RNP 4 or higher. Where necessary and appropriate, they may be based on RNP 1.

Hence the answer 0.3