Requesting a detailed break down of fees is a reasonable request.
Send them a registered letter explaining you are referring the invoice to Colegio de Abagado for arbitration. http://www.icam.es/web3/cache/index.html
There may be a bar association for the province where the work was carried out and you may receive a better response.. Send all the papers you have, again by certificated mail with a letter in Spanish explaining your complaint. If necessary engage another lawyer who is willing to take on these cases, to do it for you. Overcharging by Spanish lawyers is a serious and common problem.
Sit tight and ignore any threats