Sorry all not been on for a while but have an issue on my Libra as It’s been off the road in storage for 3years due to extension works.
Basically I did my normal checks and started it up. After a while I noticed the temperature was getting up around 100 degrees. Previously it would struggle to get above 70 degrees. I whipped off the water pump to inspect which looked brand new inside so went for the thermostat which is under the inlet manifold on my Honda engine “nightmare location” as it took s hours to get to it. Either way tested the thermostat and it also works perfectly. I’ve not altered anything from previous so was a little confused not to find a fault
I have two bleed nipples on the matrix under the format clam and have noticed the water is not bleeding through particularly well and mostly stops coming out the bleed nipple. I believe I have an issue with the heater matrix because I also can’t get the heaters warm either. I’ve never played with the heater matrix but it does look like it’s a pain in the backside to remove. The question is, is it normal to get a faulty heater matrix and if so how easy is it to remove and where can I get a replacement or something better?
I am also tempted in bypassing the matrix temporarily to see what happens to whether it improves

on a metro that gtm is based on water flows through heater all the time and heater is controlled by a flap that blocks air flow over the matrix to get cold air and needs flow of water for rest of cooling system to work properly on a civic there is a valve that controls the water flow to regulate heater temp so on cold setting there would be no water flow and the engine would not over heat so i don't think the matrix could cause the over heating as the engine could run without the flow through it as it would in a civic. The problems you are having bleeding it maybe due to air getting into the system when the water pump and thermostat were taken out or could be part of the original fault is the engine overheating or could it be a fault with the gauge?Does there seem to be a flow of water through the main radiator can you feel the water flow through the front to rear cooling pipes maybe the main radiator has blocked internally restricting flow due internal corrosion while is has been stood ?good luck with finding fault

I checked the my gauge and it works perfect, checked it also against my temperature gun to confirm as a double check. I also followed the flow of water temperature with my temperature gun allng the pipes so is flowing fine.
The radiator pipes are both correct temperatures on at the front so isn’t a cooling issue but it all seems to point at the old heater matrix with flow. I might tackle this, this weekend to flush it through perhaps

It seems odd that if the main rad flow and return, pump and stat are working fine you'd get overheating. The heater should be on a separate loop and have a minor cooling effect (none when isolated). If water is flowing properly to and from the rad then surely either the rad is blocked (so water is taking a bypass loop) or air isn't flowing through the rad core.

Yes there is two separate pipes feeding the heater matrix, I haven’t taken the car for run yet which might explain some of the issues I’ve had. Also no MOT so is a little difficult as well. I’ve seemed to have got it stabilised around 80 degrees on tick over after 45minutes running. Will fix the fuel leak issue and put it in for the MOT

Looks like I’ve over thought everything, took the car for a spin this morning and heaters work perfect now. Temperature running is back to normal low of between 60 and 80 degrees in this heat.
Car runs like a dream again