more info needed - presuming you're running a K-series engine? Is the thermo located at the water pump or is it remote? Have you blown down the bleed tube to the inlet manifold to ensure the bleed valve is open? The bleed screw you are talking about is the one on the coolant rail that runs from the rad hoses under the air box & round the back of the engine to the old thermo housing - if I take that screw out & remove the cap from the expansion tank then coolant (and any trapped gas) ****es out.
Gurgling/whooshing in the heater matrix definitely indicates air in the circuit, either not expelled when bleeding or getting in through a failed seal. You've got to try & get coolant coming from that 8mm bleed screw - take off the cap from the expansion tank, take out the bleed screw from the heater return pipe & remove the 8mm bleed screw from the coolant rail & keep topping up in the expansion tank. Gravity should do the rest. If this doesn't work then it's very likely going to be more serious/expensive - maybe the coolant channels in the engine are blocked up, (e.g. maybe someone mixed green & red coolant, maybe someone chucked one of those leak sealants in there and it's blocked up the channels). Anyhow, if coolant isn't circulating you may well have to consider that the HG has become compromised.
I took the head off my K series engine this week (Freelander) - the head had been skimmed beyond tolerance & refitted by a previous owner. The HG I took off had de-laminated very badly indeed. You can get a brand new genuine LR head for £310 so if you are mechanically skilled replacing the head & HG is not expensive.
Cheers
Simon
