Had 5 schengen in the last 2 years and all of a sudden the Spanish required the Thai Embassy to stamp our marriage cert. We did have to make an appointment and the cost was £10 cash. What language is your certificate in? The Spanish Embassy do not return the certificate so make a good copy ( Italy could be the same)...tell them it is the original and hope.
Whilst BLS dont charge for a visa it still cost me £100 Thai Embassy , visa application, visa collection, 3 trips to London, train and car park and handling charge Oh and a coffee when you get there. Cheaper to stay in England. Good luck