Thanks for the pointers guys - the Value vs Value2 argument got me a different set of search results which helped me realise what the answer is. Incidentally, the Value property is a parametrized property, which must be accessed through an accessor in C#. These are called get_Value and set_Value, and take an optional enum value. If anyone's interested, this explains it nicely.
It's possible to make the assignment via the Value2 property however, which is preferable as the interop documentation recommends against the use use of the get_Value and set_Value methods, for reasons beyond my understanding.
The key seems to be the dimension of the array of objects. For the call to work the array must be declared as two-dimensional, even if you're only assigning one-dimensional data.
I declared my data array as an object[NumberofRows,1]
and the assignment call worked.