I finally managed to run Purchase Lookalike Ads. Thought it would be the best lookalike audience of all, however Add To Basket Lookalike Ads performed so much better, even interest based ads did too.
Purchase Lookalike Ads = £16.56 per purchase (reach: 3M)
Add To Basket Lookalike Ads = £6.62 per purchase (reach: 300k)
Interest based Ads = £8.16 per purchase (reach: 3M)
Anyone know what might be going on here?