It's too late to ask for a replacement boiler (under the sale of goods act) so it would have to be done under the warranty again. It seems that there may be another problem causing that component to fail so it might be worth pursuing this with the manufacturers and give them the history. Hopefully they should look at it sympathetically and offer some help.
Your complaint rests with the company that supplied it, rather than the plumber who fitted it ( unless he works for them).
Rod