Hello Stuart
the 'sen i" and 'set ti" bits are abbreviatons from “tasen i, taset ti” etc. In the North you would hear “taswn i” or 'swn i" for If I were…
The other forms are:
taswn i
taset ti
tasai fe
tasai hi
tasen ni
tasech chi
tasen nhw
If you are more interested in these forms, you could get more information in this Taswn/tasen thread.
I hope I could help you.