It takes too long for the engine temperature gauge to warm up and for heater to start blowing hot air.
My first thought is that the thermostat might be permanently open and the radiator in the circuit instead of initially closed off.
Is this sensible, or are there any other ideas first?

I've moved your question from Other Jaguar Models / Concepts / Replicas to XJ6 Series I,II & III forum.

Members here with the same model will be able to help.

Graham

I'd go for the thermostat first. Start with simple cheapest things!
But in this weather, the old lump will take some time, especially if it's got the engine driven fan.

Thanks - You are right about the fan - it is fan belt/engine driven. Is it safe/sensible to put a screen, cardboard sheet perhaps, in front of the radiator, especially on only local shortish trips?
Robert