Op from your second post on this it would appear that this has already been through some sort of internal process in the bank given that mediation and FSO is being mentioned.
If it is the case that it has got to that stage then the bank should be able to give you a written breakdown of the interest rates applicable, the dates they were applied and what interest was charged. They should also be able to give you a copy of the initial loan offer letter outlining the interest rate and what terms apply and how they calculate the interest rate.
If there is no specific detail for how they calculate the rate then it is just a variable then there is nothing to stop them increasing to any rate regardless of euribor provided it was not usurious.
I would ask the bank for details of their personal loan interest rates for the period, see how they compare and ask the bank to explain any differences. If your rate is significantly higher than their normal personal loan rates then you may have some strength in challenging them.