You could switch the hasProperty and the allOf matchers.
verify(orderService).createOrder(
org.mockito.Matchers.<CreateOrderEvent>argThat(
org.hamcrest.Matchers.<CreateOrderEvent>hasProperty("details",
allOf(
hasProperty("dateTimeOfSubmission", notNullValue()),
hasProperty("name", equalTo(CUSTOMER_NAME)),
hasProperty("address1", equalTo(ADDRESS1)),
hasProperty("postcode", equalTo(POST_CODE)))
)));