What is causing this Leaf discolouration overnight after a lot of tent work yesterday

Not really a huge problem and very simple to solve it. It is clear that you have a calcium imbalance, probably due to two main issues : at the moment perhaps the PH rate is very low (I assume that your PH rate is around 5.8 up to 6, and this rate is not the best for the calcium integration); second measure that you should start to provide right away is linked to the calcium available. You can solve this imbalance in not more than 36 hours, by raising the PH up to 6.3 to 6.5, after that you can provide some CalMag or solve some Bone flour in your watering process and to facilitate the integration of calcium you can put some Kelp (Ascophyllum nodosum) on the watering solution both by fertirrigation or spraying in foliar sprinkling. You have one of the best liquid kelp supplies on the Cultivers brand (on Amazon) and kelp is a kind of the magic solution for almost every stress or imbalances that you can pass trough your grows. So as I told you you can solve this problem (easily and fast - within not more than 2 days): raise up PH to 6.3/6.4 ; use a little CalMag (important...not a lousy brand but a good one, and preferable a mineral one) and finally use kelp as a stress reducer and as catalysator to calcium integration (the calcium normally is available in much products, but it's vertical availability and the way that is integrated is hard. The last option and once for me was the most optimal (but almost desperate) solution was using what a botanical PhD friend of mine, though me to do as desperate measure, that was to pick tomato seeds, let them fully dry and after that using a Moulinex grinder (the ones that you pull 1-2-3) and make a tomato seeds flour. That flour has a strong compound of calcium, the calcium oxalate that is very solvable in water and highly absorbable by the root system. So I used that once (not on cannabis, but on peppers that have revealed nasty lack of calcium along their growth) and solved my problems, very very fast. So try this and you problems with calcium shortage will be end in few days.