I am pleased to report that, suddenly, my fermentation kicked off over the weekend!
After trying various things such as champagne yeast, decanting, starting a fermentation with shop-bought apple juice and mixing it in, it suddenly started over the weekend! It was a very vigorous fermentation as well. Because I'd added shop juice I have two fermentation vessels - one smelt awful - 'rhino farts' is the phrase I've heard people use. I'd added more yeast nutrient to address that, and a penny from 1981! I added some of the violently fermenting (It was literally boiling!) one to the rhino farts one in the hope of improving it.
That was a couple of days ago an the fermentation has slowed right down now. I will probably take hydrometer readings later in the week - I suspect it will be time to bottle it soon. Who knows what it will taste like? It might be awful!