Firstly, a couple of questions - what colour is your coolant (not the water, bit - I can guess the answer to that

) - there are basically 2 common types of coolant - Blue stuff and red stuff (Methyl based, and 'OAT' - Organic Acid Technology)....
... If you (or someone else) has used an OAT anti-freeze/corrossion inhibiter (Red stuff) then it could be that which you have found - it is great for it's job, but unfortuately not always compatible with the (older) elan cooling system.
It only needs changing every 5 years instead of 2, so lasts longer (but costs more) - but crucially, it doesn't like certain materials as used on older cars - great on modern motors with Aluminium and plastic tanks, but not great with some brass/bronze bits and pieces like we have.
It is possible that it has caused a failure somewhere, possibly at he radiator end tank - a common problem area.
However, if you are using Blue antifreeze, this isn't going to be the problem!
The only other 'red' stuff I can think of is the Power Steering fluid, and there are pipes and pump in the area you have found the leak. If you remove the lid from the fluid reservoir, it will have a dipstick built into the cap, and you will see how much you have - but if it's as big a leak as you say, I would expect the system to be empty.
I can only suggest that you wait until it's dry and get under for a squint - alternatively, can you remember if the leaked fluid was water soluble? - Power steering fluid isn't (very easily).
Please don't be tempted to try the oft-advised 'taste test' - OAT is poisonous, and Dextron fluid tasts horrible
