I phoned up the customer support number (0800 50 55 55) to find out what this meant:

1) Grown in the West Bank area administered by the Palestinian Authority,

or

2) Grown under West Bank area administered by the Israeli Government.

The answer was 2 - it is grown by an Israeli farm, therefore almost certainly from land directly stolen from the Palestinians through illegal house demolitions, forced removals, military rule, etc.

Apparently it is "In-line" with EU regulations, that produce grown on stolen land is labelled "West Bank". Personally I think this is far too ambigious, but I know that anyone who cares about issues related to the occupation and subjugation of the Palestinians should avoid this product - and possibly any others labelled "West Bank" - from now on.