Thanks Scott for that.
That sets my mind at rest. I was just thinking that the 'nodule' was the point at which the anal canal and sphincter which surrounds it was cut and it seemed a bit short, i.e. not much more than an inch.
Now I can continue with the Kegel exercises and hope that the sphincter muscles will eventually be strong enough to hold it all in, whether the stool is well formed or runny. I read somewhere that diarrhoea does not have to mean incontinence.