Thanks for the response, my plants are indoors and two of the ones in question (the two with blemishes on the new growth) I've had for about 3 months and the marks developed recently for no apparent reason so I am not sure about mechanical damage and it doesn't look like something the sun would do. Out of the other two, the one with the rough looking streak across the leaf did make me think of sun damage, the other one has yellowish pitting which didn't look like sun damage to me but then I googled some more images of sunburn and found some that look similar so that's feasible.
The phals are facing a south western window but they're not on the windowsill, they're on a bookshelf a few feet away from the window. I live in England in a city so on the rare occasions when the sun does shine, it is not very strong nor are they exposed for long, but to be fair we recently had a heatwave and my new purchases caught the tail end of it, it got as bit hot in my room (which happens to be an attic room to make matters worse!) So I suppose sunburn could explain the two new ones but I am still worried about the other two with the blisters and strange marks on the new growth, I really can't think of anything mechanical that would have caused that
I spent too long googling what could be wrong with my plants and after reading about too many devastating diseases I am worried for my babies