If the old passport is lost and you have no physical access to it (only copies), you are typically required to apply for a new DTV visa since Thai immigration does not transfer visas or update the passport number electronically. The advice from multiple sources including the Royal Thai Consulate and Thai Visa FAQs stresses that the visa stays with the original passport and cannot be reissued to a new passport directly.

A cover letter should clarify that you live and work in Spain during the summers, specify why you are providing Spanish bank statements, and mention your UK/English bank statement if you have one with your home address on it. Include details explaining your ties to both Spain and England, and your reason for travel to Thailand (such as Muay Thai training)