Hi Greg, this looks like a 'classic' Commer feature !
Who's not been squeezing their bulb ? [:0]
If you look at the lowest apex, between the 2 heater flow 'controls' (ie. flaps / doors), there's the stubby remnants on the outlet pipe from the heater 'airbox' itself (between the heater unit & the front grill). This should be attached to the vertical rubber tube just below it
Basic principle is, that any water entering the front top grill inlet from outside should drain away before getting to the heater unit.
Below the vehicle the rubber drain tubing end in a 'bulb' terminal which is supposedly to prevent fume/insect ingress into the cab. Unfortunately over time these drain bulbs harden and get full over cr@p and block up which unless they're squeezed regularly (& they're in a god awful location) results in the whole drain backing up with water and eventually rotting the airbox etc away.
With luck your airbox might only need a small repair on its outlet pipe and a new, softer drain pipe to restore its original function. Afraid it looks like some water has been getting into your heater unit at some point though, judging by the rust along its bottom edge.
Suggest further investigation is required around the airbox, heater unit & front grill inlet.
If the heater unit is internally rotten, it can be replaced, some NOS come up from time to time for not much but some on the forum have replaced their original heater unit with one from a mini.
Big question here, is whether your airbox (which mates the heater unit to behind the front top grill inlet) is beyond repair - otherwise some more creation solutions are required
'Hunny' our Commer Hi-Tor by TorCars