Many thanks for all of your thoughts. I confess I'd forgotten that I'd posted the question!!

The engine was replaced about 2 years ago (I wasn't the owner then) after coolant problems wrecked original.
As far as I can tell the heating system has not been modified in any way. They simply slapped in a new engine and connected up the pipes - and the rear heater is still intact and has not been modified in any way.
The only way you would know there was different engine in it is by looking at the bill (!) and the fact that the existing pipe elbow does not have an extra output as the bleed point.
If the Ranger cooling system did have a bleed point in a different place - I certainly haven't been able to find it - and as you say - with only 1 heater - I'm guessing it wasn't necessary in a Ranger. To save a whole load of grief which it looks like it'll be to fit this elbow - I'm considering simply creating a bleed point by T-ing off of the feed to the turbo. That hose is pretty short but looks possible.
I think removing the rear heater and capping A/C pipes is an excellent idea. That would certainly remove some points of failure.
Rob