I'd guess it was a dodgey coolant filler cap, theses are notorious for failing...... get 2 and always carry a spare.
I'd say you were lucky enough to shut it down before any damage occured.
Look or any sign of oil in the coolant or coolant in the oil.
Globs of what looks like tockley snot in the coolant will be oil.......not good!
And snooty looking shit on the dip stick and the under side of the oil filler cap........also not good!
A must do for these cars is fit a coolant level sensor kit
The one pictured is the one I use, the
Brown & Gammons kit,
very easy to fit and use.
there are others out there buut IMHO this is by far the best.
The indicator goes off when your coolant level drops by @ a litre or so, this buys you time to shut the engine down and look for the cause before any risk of damage occurs, giving you much piece of mind.
Don't whinge about the cost of the kit, compared to the cost of a new engine.....it's nothing!
Anyone with an MGF / TF not using a coolant level sensor is insane and needs locking up for their own good!
Beg, buy, borrow, bludge, hire or steal a compression test gauge set
and do a compression test. The important thing isn't so much the actual gauge reading as the variation betwen readings of the varoius cylinders, you don't want any more than about 10% variation between cylinder pressure readings.
Whilst your doing this inspect the spark plugs (as you will have them out to do the test) if one of then is much cleaner than the others then that is probably going to be the cylinder with low compression, it's probably clean as coolant leaking into the combustion chamber will have steam cleaned the plug.
The next trick is to beg, borrow, etc. a radiator pressure test gauge set
Now as per the instuctions provided wit the kit replace the filler cap with one of the adaptors and pump the pressure up to @ 15 psi as indicatored on the pump gauge what you want is the pressure to stay at the same level over a period of time. If the pressure drops you system has a leak.
While the system is under pressure do a bit of and inspection... radiator, hoses pipes etc. to see if you can see any coolant leaking out.
Now here in Oz we don't have Kseal, but I assume it a compound that you tip into the cooling system that will
sometimes fill a gap in the head gasket or fill a crack in the head or block,
We have various products that do much the same, Irontite, Chemiweld, Bars Leaks etc.
I doubt these will do much harm, I doubt they will do much good either. I would flush the system.