[mclarkuk1]

Im assuming its a magnesium deficiency.

[Trillium]

Yes, this is magnesium deficiency. It's made worse if you've used a higher potassium fertilizer (third digit in commercial fertilizer numbers) which binds magnesium from being absorbed. If you haven't used a fertilizer, then there simply isn't enough magnesium in your soil mix.

Because it's now illegal to advocate home remedies in the UK, I can only tell you what I use, which is 4 Tbsp epsom salts to 1 gallon water and a drop or two of washing up soap, stir, and spray or water over leaves and soil. In a few weeks I notice the difference.