Jimbofinn,
An interesting post and you did well to have your claim paid. It's unlikely to be covered under the standard Spanish policy wording. The usual basis for paying a claim is following the occurence of an insured peril - a fire, lightening, explosion, water damage etc etc.
Some policies can have a wide underground services extention. Ot they might have paid the claim, perhaps wrongly, if you you had the "accidental damage" extension.
You don't specify the exact cause of the loss/damage, which is all important, but I reckon most insures in Spain would have turned your claim down. I have known them to cough up as they don't know their own policy wordings (technical standards are quite modest), sometimes for PR reasons (if it's a small amount) and occasionally just to "keep the peace".
For sure you generally need to have a robust approach with most insurers in Spain, as clearly you did - pity Teracota1 did not do the same.