It's possible the tapioca flour in the Asda mix is causing the grittiness in your cakes. Tapioca flour is a starch and doesn't dissolve well on its own. If not used correctly, it can leave a grainy texture.

Solutions:

Reduce Tapioca Content (if possible): Check the Asda flour mix ingredients list. If tapioca flour is listed near the bottom, it's likely a minor ingredient. You could still use the mix, but consider reducing the overall amount used in your recipe by a small percentage (maybe 10%) and see if the grittiness improves.

Alternate Flour Mix: If you'd rather avoid tapioca flour altogether, look for a different gluten-free flour mix that doesn't list it as an ingredient. Many brands offer all-purpose gluten-free blends that work well for cakes.

Using Tapioca Flour Correctly: If you still want to experiment with tapioca flour (it has good thickening properties), here are some tips:

Mix with Wet Ingredients: Always whisk tapioca flour with wet ingredients like milk or eggs before adding it to your dry ingredients. This helps prevent clumping and allows the starch granules to hydrate properly.
Cook Briefly: For some recipes, briefly heating the tapioca flour mixture can help activate the starch and improve its texture.
Experiment with Other Flours: There are many gluten-free flour options besides tapioca. Explore options like almond flour, coconut flour, or oat flour, depending on your recipe and desired texture.

Alternatives to Asda Mix:

Here are some suggestions for alternative gluten-free flour blends:

King Arthur Gluten-Free All-Purpose Flour
Bob's Red Mill Gluten-Free All-Purpose Baking Flour
Cup4Cup Gluten-Free Multipurpose Flour Blend
Pamela's Products Gluten-Free All-Purpose Flour Mix
Finding the Right Flour:

There's no one-size-fits-all solution, so it might take some trial and error to find a gluten-free flour that works best for your baking needs. Reading reviews online and checking recipe recommendations for specific gluten-free flours can help guide your search.

By understanding the role of tapioca flour and exploring alternatives, you should be able to achieve the smooth, delicious cakes you're after!