I was told by the GP that it was: “very unlikely to be anything sinister”, and that she thought I had a slipped disc.
So I was referred initially to a neurosurgeon (not neurologist - subtle difference) - because it was expected the problem would turn out to be mechanical - disc pressing on the spinal cord.
The neurosurgeon ordered an MRI - of spine only - to look for the dodgy disc. Scan came back - no slipped disc.
The consultation was almost over, and he was just about to discharge me, when he looked one more time at the screen, and said: “Wait! I want to know what that is!” And indicated an area of the scan that looked very slightly misty, and was about as wide as dental floss!
I’m sure he knew at that point what he’d seen, because, in a sudden change of heart, he said: “If you don’t mind, I think I’m going to refer you to a colleague for a second opinion.”
Of course, I lost no time in Googling the name of the colleague, and discovered he was an MS specialist. And the rest, as they say, is history.