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There should be a vacuum line (with an anti-return valve in it) running from a t-piece in the vacuum line between the inlet manifold and the brake servo, to the vacuum switch mounted on the side of the heater box. You can find this switch in the passenger footwell (in an RHD car). A second vacuum line runs from the vacuum switch to the heater valve mounted on the exhaust cam cover. The vacuum switch is activated by the heater slider in the centre console. Not all J-Hs have the heater valve, it was fitted to later cars to reduce heat soak into the cabin. If the vacuum pipes are connected the wrong way round at the vacuum switch, the heater valve will not work. Try connecting them to the vacuum switch both ways round. The heater valve is a Jaguar XJS part, easy to find on eBay and elsewhere. Hope this helps.
Best wishes,
Nigel