hi sage,
i've recently changed my thermostat housing on my disco 300tdi L reg,as a previous owner bodged the bleed cap,
knackered all the thread and no matter what or how i tried i couldn't stop it leaking.
after changing the housing, i replaced the thermostat with a new one,
i toped up through the bleed screw hole untill water level came up,
screwed the replacement copper cap and rubber seal.
started the disco and ran up to temp,
once at temp opened the bleed screw, some air came out and ran with water,
now heater nice and warm constantly, temp ok, and had no problems since.
hope this helps