Looks like you have a strong case on the face of it Jim
The only mortgage document that you have signed states that if you moved to a fixed rate (which you did), you would then revert to an ECB + 1.1 tracker and nowhere does it state that you will move to a variable and PTSB never informed you of this at the time?
What has your broker said? I would give them a call and let them argue the case on your behalf, banks regularly try to wangle their way out of such agreements.