Roy,
I agree that the price to change for a double socket is a massive rip off. I would imagine the bronze type sockets will be available to buy in the DIY store. To make a single into a double would mean cutting the tile (if it is on a tiled wall) and cutting out the hole such that a double box will fit. Below is how the wiring is in our marbella at Pinada golf also built by san Jose.
Where you need to be careful is which sockets you would like to change into doubles. The Spanish in my opinion are light years behind the UK in the way they wire their houses. They do no use ring mains on socket circuits as we do in UK. A ring main means that at the last socket a cable is returned to the fuse board. This means that the current used on the circuit is shared on the first and last cable. In Spain they do not install the last cable so the circuit is what we call a radial circuit where all the load is on the first cable from the board.
The sockest in the kitchen will be fed from a 16 amp circuit breaker and other SOCKET circuits will also be from a 16 amp circuit breaker. Where you need to be careful are the sockets in the bedrooms next to the bed. These sockets are fed from the lighting circuit which will be 10 amps breaker.
Hope this helps.
Ian