From 2 experiences of HG failure on Mk1's, both same symptoms
NO oil in water or vice versa.
Continual slight loss of coolant
Slightly higher temps especially in stationary traffic
Gurgling sounds in heater matrix especially at tickover.
Slight misfire accelerating from cold start.
Much larger difference than normal in level of coolant in expansion tank when hot and cold.
The heads were removed and both showed evidence of leakage from cylinder to water jacket on no.3 cylinder.
Of course, both of these were the start of failure , not sure what happens if driven to the death !