I only know one person who went down this route over a very bad car accident in the UK involving the whole family and the car, and before someone jumps in and tells me this is Spain not the UK, yeah I worked that one out, anyway maybe these people work in the same way.
The costs that were laid out to proceed were agreed before any action was taken, the chap won the day and was awarded money due, but a couple of weeks later he got another invoice for quite a few hundred pounds more for an insurance policy that if the 'No win no fee' company lost the case, it seems they buy a policy to cover their losses, this person won the case but still the company wanted this extra fee because they paid for it in the event of losing, no mention of this was made on the very first agreement date.
No doubt your case is worth quite a few more €'s or £'s then the car accident, and perhaps this 'Extra' policy rises the more the claim is, I don't know, and didn't even know they did this, but it does seem to make sense due to what lawyer will ever lose money?
Could be something worth looking into.