Also, will Ulster Bank facilitate splitting the mortgage in two?
Your first point against fixing ( The banks have a better idea of future rates and build in a margin for fixing, so they are not usually good value )
I think is the most important.
In my experience and that of poeple I know, I have yet to see anyone save money by fixing for any period.
Obviously some do, and there is also the security of having a standard payment each month to take into account.
The personal circumstances of the mortgage holder should be the most important factor in deciding.