Why is my heater now working?

Last week I had the following work done on my '94 300Tdi Disco,

Engine oil
Oil filter
Antifreeze
Head skim
Head gasket
Head bolt x4
Injector seal x4
Valve cover gasket
Intake gasket
Water pump
Pump gasket


About a month ago the heater in my Disco stopped blowing any hot air even after a 30 minute drive, I presumed thermostat had gone and so was planning on getting a replacement tomorrow but driving home from work a miracle, hot air! Looking at above list can anyone shed any light as to what
has caused my previously defunct heater to start heating again?

plain and simple. you now have coolant in your coolant system.

If you took the water out of a normal household radiator would it heat up?

Cheers Ratty, I now feel a bit of a numpty for not realising the most obvious reason!