Hello once more, James. I have been doing a little bit of Googling, and are available throughout one thing known as the “D&D Necessities Equipment,” which features a new guidelines variant particularly for one participant and one DM (the “Sidekick” variant). It is at the moment on sale on Amazon (£16.49! Cannot say fairer than that!) Sorry I did not point out it sooner, however I am not very conversant in the 5E product line. May be a greater place to begin (however cling on to the Starter Set, when it arrives – you may be wanting the journey and further cube).
Googling “one-on-one D&D” yields some attention-grabbing discussions; have a great go searching and skim at your leisure!
All the most effective.