I don't think there is a service kit as per. When I ordered the parts from Land Rover to do my Range Rover (same 6 speed box) on the receipt it was just invoiced as a sump and oil separately rather then as a kit.
You'd need the plastic sump (only buy genuine ZF as others can leak) and the oil.
If I were you I'd also drop the valve block and replace the bridge seal and the little rubber sealing dowels above it as they can compress and stop sealing properly over time.
The other option you have rather then opening the box up is to find someone with a machine to do a LOW pressure flush on it. Much less hassle and less mess.
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