Hi . My outdoor fatsia japonica has suddenly developed black edged dying leaves and is looking really sick . I have found several leaves with tiny black insects on the underside too . ( magnified image below ) Are they both connected and can I rescue my lovely plant ? It is about 10 years old and 6 foot tall . I would be so grateful for any advice

It looks like you have a couple of unrelated problems.

The brown leaf tips are the result of either incorrect watering or too much direct sunlight. Have you changed something outside, like having a tree cut, that would result in your Fatsia being exposed to direct sunlight? If not it's a watering problem: it's not easy to gauge watering needs with gravel on top of the soil.

The insects look to me like the sad remains of ants who got stuck to the leaves and died, most likely stuck to the honeydew they were trying to gorge on. Honeydew is the byproduct of aphid and mealybug feeding and while I do see plenty of feeding damage in those magnified images, I don't see the cause itself. Maybe a better inspection.

Thanks so much . We have recently had a patio built around the plant - previously it was surrounded by soil and gravel . It gets very little direct sun as it is north facing . I have removed all diseased leaves. Am I best to just sit back now and cross my fingers ??

So it's most likely a watering issue.
You may want to consider putting a soil hygrometer (with a long probe to get past the gravel) next to it to see when it needs watering. Something like this: https://www.amazon.com/dp/B014MJ8J2U/

Also keep an eye out for pests by regularly checking the underside of the leaves.