It sounds like a coolant loss issue to me - when the level is low, the heaters will feel 'lukewarm' as they're quite high up in the coolant circuit.
A good place to start with a 1.8 (assuming you have the plastic inlet manifold) is the inlet manifold gasket - these go hard with age and start to leak.
Sometimes they leak into cylinder 1, so there's no external indication you're losing coolant!
We have the revised Viton gasket designed to last a lot longer here:
https://www.dmgrs.co.uk/products/k-s...sket-lkj101110
Another source of coolant loss is the O Rings in the coolant cap; they flatten with age, allowing steam to escape at higher pressures when on the move.
Again, hard to spot as the loss is often gradual.
You can get replacements here:
https://www.dmgrs.co.uk/products/rov...to-fix-esr1480
These issues probably account for a good 50% of K Series coolant loss episodes we're asked to help diagnose these days - very common, but thankfully both relatively simple repairs.
