Posting in case it helps someone else.
I suffered with mouth ulcers for years, tried lots of SLS free toothpastes, eliminating foods, etc. Nothing made much of a difference until I randomly picked up this little gem in Lidl.
Dentalux Sensitive Plus toothpaste.
I haven't had an ulcer since I started using it months ago. Amazing stuff. If you have the same problem give it a go!