try taking viscous fan off and blank off the rad the cold air will make it take longer to warm up must say my disco aint got the best of heaters
 
Does the viscous fan work properly, or is it on all the time? Better still take it off - mine has never been on and temp control works fine.
 
I blank my 200tdi radiator off especially the oil cooler bit on offside. I have about 10 % only exposed and no fan at all. New thermostat and she still takes ten mins to warm up to normal. Stays there when warmed up though.
 

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