Thanks guys, I’ve tried most of these things. I’ve watered with Epsom salts once and used it in the foliar spray. Maybe it takes some time to show. I’ve also moved the plants back from the window as it’s getting sunnier. There has been some very slight fading on the odd leaf close to the window.
The plants don’t get very warm. We live on the SW tip of England in an extreme maritime climate. It rarely gets into the mid twenties centigrade here in summer, the effect of the sea and prevailing SW wind all the way across the Atlantic is so strong. Plus when it’s sunny, the windows above the plants are open and letting in cooler air.