Hiya Mick
Sorry to hear you got caught, but I have warned people before about this, although it may have been in the Costa Blanca forum.
No consolation, but he is not the only one, it is the policy of Renfe and therefore applies throughout Spain.
You must have a ticket before you board the train, otherwise you will be charged an extortionate amount.

Be warned indeed.!

Sanjiiiiiiiiiiiiiiiiii

Regular users of those ticket dispensers may notice that they sometimes take €20 banknotes and sometimes they don't. The reason depends on whether there is enough change available in the machine. As the amount of coin available to give change drops, it disables the €20 note, then the €10 note and finally the €5 euro note when the available change has all gone.

Coins put in the machine to buy tickets are recycled so, in theory, there should always be change available but I guess the ticket dispensers at the airport take more money in banknotes than they do in coins, hence the problem of not having enough coins available to give change.

The €50 euro note is never accepted.

A.M.

I was on the Train once and some people got on a the Airport but the ticket machine was broken so they couldn't get tickets, they paid the high ticket price on the train but the conductor made a note on their tickets and said that when when got off the train (Arroyo stop) just hand the tickets over at the desk explain they had to pay the high amount as the machine was broken and then they would be given back the difference.

I see what you mean Sanji, this from friends of ours to the local paper on the Costa....
...
Train tickets
My wife and I recently boarded the train from Los Boliches heading for Benalmadena. The automatic ticket vending machine at Los Boliches was out of order and stated so. In view of this my wife and I boarded the train with several other travellers, assuming that, as the fault was with Renfe, we would be allowed to pay on the train.
We were - however we were charged 5.95 euros each for a single journey which with an automated ticket would have cost 1.15 euros. The other unfortunate travellers nearby, all elderly, and not a bit how I would imagine liars and fare dodgers to be, were treated similarly.
The ticket collector, who looked intelligent and honest, apparently had no discretion in the matter. Renfe had not empowered him with such common sense.
In other words, unless you are in possession of a valid ticket, don't board the train. Additionally unless you are fluent in Spanish don't attempt to complete the Renfe complaints form.