“Factor in the greatly increased cost of a second hand Leaf (2014 at around £8K) and it would take me about 6 years before I started seeing any savings, and by that time I'd probably need to replace the batteries.”

Please stop thinking the batteries will need replacing, they won’t, they are outperforming all expectations and 10 year old batteries are still at 90% plus state of health.
If the car is kept for several years the overall costs of ownership are way cheaper than ice cars as there’s no engine, exhaust, cat, radiator or other components to need replacing.