How do I make this no bake tart filling less runny?

Welcome to the site Elle! This tart filling is a custard as the thickening agent is egg yolks. If it isn't thickening up enough then one of two things is happening:

1. The mix isn't being brought up to temperature. The recipe's instructions aren't very good because it talks about a time rather than a target temperature. Egg yolk will set at about 70°C (160°F), if your heat isn't' high enough you may never reach that temperature. I suggest using a thermometer when making custards because it takes the guesswork out of it. If you reach the target temp and it hasn't thickened up enough then you don't have enough setting agent, which is #2
2. Not enough setting agent: A few things could be throwing off the ratio of egg yolk: your mango could be less thick, or you could be using smaller eggs than the recipe calls for. This is common when people in the US make UK, Australian and New Zealand recipes as US eggs sizes are smaller - medium UK/ANZ eggs are large in the US. Most UK/ANZ recipes call for large eggs, in the US that would be XL or even Jumbo

I would first try gently cooking down the mango a bit to thicken it up before adding the eggs. This takes more time as you'd have to cool the mixture before adding your eggs to it so they don't scramble and ruin it, it will also intensify the flavor. If you don't want to do that then add another egg yolk, or use larger eggs. Then I'd use a candy thermometer to monitor the temperature to make sure it gets to the target. It should thicken up noticeably when it gets there. If it still doesn't add small amounts of cornstarch slurry (mix with a bit of your extra cream), holding the temperature there until it thickens up. Don't go overboard or you'll have too hard a filling when it cools!