You need to change the potting mix. Potting soil is not good at all. Look up 5-1-1 to get the ingredients. I recommend potting it in that, removing as much soil as possible from the roots, and feeding with Foliage Pro at least once a week.

I used a soil specifically formulated for citrus plants, rather than a general potting soil.

I dont think this is the root cause.

I included the food in the pic as well - npk is 5.6-4.7-8.6

If you are sure the soil wasn't correct I can change it, just double checking as my gut says it's the hard water.

Happy for any advice

Too high pH of the soil or water will inhibit uptake of some minerals that will lead to the symptoms you have. I would also change the fertilizer you are using, as recommended above; but in the meantime just give it a little extra.

Mix the Foliage Pro with a little white vinegar. You can buy cheap aquarium ph testing kits to get a rough idea. The ph of your watering solution should be between 6-6.5. Test it after adding the FP and vinegar as the FP drops the ph on its own. The first time I did it, I tested once after adding the FP and then again after the addition of vinegar.

You should also buy some Oscomote plus to mix into your mix. As far as the mix is concerned, even though it may say for citrus, it will not be nearly as good for your plants as what you can mix up yourself. I was reluctant at first and resisted also, but it is really quite simple and makes a huge difference in how happy your trees will be. Keep in mind (and you especially with hard water) will need to water thoroughly to wash away deposits. If your mix does not drain freely, you will not be able to do this well. I like the 5-1-1 mix because it is lighter and I also find the ingredients easier to find. I mix into the mix a scoop of the Oscomote plus for good measure and water with FP and a little vinegar.

One question for you...where do you live? What zone? If you are bringing the tree in for winter, the mix will be more important than if it stays out all year.

Do you have a water softener? If you do (and it is quite common with hard water), do not water your plants with water that comes from the softener; it has salt added, which will eventually kill your tree.

I did not know that John, but it's good to know. I don't have hard water though.

Water softeners do not add salt (NaCl). I

they switch the Mg or Ca with Na or K depending on the system. Salt is not added to softwater in this process. Just use potassium chloride water softening pellets and you aviod sodium.