Re: Heater not working, fluctuating temp guage
I would see how much hot (not boiling) water I could get into it.
With anti-freeze if you've got it.
If it comes out again you can see where its leaking.
If not much goes in start it and idle for a while, see if level drops.
With luck it will be OK, but best take it to garage anyway.
Also check dipstick and oil filler cap for signs of water.
Check cylinder head/block join for leak (if you can see it).
A leaky head gasket should be audible too.