Had same problem with my 80...
Not sure of set up on UR, but having had the matrix replaced on mine and with it giving out after just 18mths, well I tried :
Thermostat replacement (located inside coolant housing)
Temp sender replacement (blue thing near plugs)
Coolant flush/change
- all to no avail mind!
Then a friend suggested flushing heater matrix independently from coolant system, and viola! Hot heater again.
Looks like you've tried most of those things though, so aliken to previous comments, sounds like your matrix is foo-bared.
