A 23 year old heater core could be so crudded up that no amount of flushing will thoroughly clean it out. The key to a good heater are lots of hot, fast flowing, coolant circulating thru the heater core. The condition of the water pump and engine rpm determine coolant flow rate. How old is the Audi compared to the XJ? Might be an apples/oranges comparison.

Ok so it could still be the heater core, sounds reasonable.
Not sure of the condition of the water pump though, I could see water pumping, but if it was the correct rate, no idea.
Will most likely try and burp her tomorrow again, just to make sure I don't have more air in there. Then, maybe I will convert to an open system over the summer, and install a new water pump.

Of course the Audi is newer and in a different league from the XJ. Only wanted to compare the heat blowing, since I wasn't sure how hot it was blowing in the XJ. Gave some indication that the XJ is not quite as hot right now, can't even remember if it was hotter before I got the problem.
But I do have a few rust holes in the back, which will suck out hot air when it's -20 Celsius outside! Gotta cover them up...

Thanks for all the help, might come back with some updates after the weekend.