Impreza UK 2000 heater issue

[JM]

When the engine is cold check the coolant level.

Does sound like it could have been an air-lock as they can be awkward to bleed all the air out, and raising the front is one way of helping do it.

Then the next thing to ponder is, how did the air get in and where has the fluid that should be there gone?

[JM]

I need to top mine up every now and then, with no obvious signs of a leak either.
Though I'm thinking of replacing the radiator anyway as I'm sure it's original and could have a small leak or two somewhere.

[vxr2010]

it can indicate a hg issue sorry to put that point in there