Ferreteria Lopez stock larger electrical boilers, but you will need a certified (in Spain),electrician to replace it.
Spanish law states that anyone attempting any electrical installation in a home other than their own, must carry certificates to prove that they are qualified to undertake such work. These certificates must be authenticated by the Spanish authorities, this is clearly in an attempt to safeguard not just yourself, but your neighbours, as, should there be a fire as a result of an illegal installation, a public prosecution will follow.
You may need to check that a larger boiler will fit inside the kitchen cupboard without compromising it, unless the boiler is situated outside.
Hope you can sort this problem out without too much expense,
Lorraine Braid. 12-3-07.