\\\or do I wander down to Beachy Head and fling myself off....?\\\
That is bloody dangerous.
The do indeed sound like aphthous ulcers.......annoying, painful and often difficult to treat. The medication that your doctor has prescribed is the standard treatment and 3 weeks seems a long time to suffer.
The cause of aphthous ulcers is unknown.
I am not sure what you expect from your dentist, but I am sure he/she will not "fob you off.
Just an afterthought......you haven't developed any ulcers elsewhere, have you? Vagina or around your genital region?
Keep me informed as to the opinion of your dentist.