What do the Terms & Conditions say? Is this a mortgage that has already been drawn down? If it's linked to the last ECB rate change then presumably you have been expecting it, so it isn't completely out of the blue and not budgeted for?
What exactly does the letter say? What specifically changed? The interest rate charged? The way that they calculate interest? Your mortgage interest relief/total interest bill? Something else? If it's a standard variable rate mortgage then they can change the margin any time they like as far as I know. As above your T&Cs should clarify.