Unforgettable Love Stories #11 c. 1977 features a Keith Chatto cover and is a reprint of the first issue of the series. The story is titled “Storm Warning”.
Unforgettable Love Stories #13 features the same Keith Chatto cover but a different story altogether – “A Very Precious Thing” - most likely reprinted from an earlier issue in the series.
I say “most likely” because this story was definitely published in the earlier Page Publications comic Confessions and Love Library #12 c. 1974, which featured a different Chatto cover, which was itself published a little bit earlier in yet another Page Publications comic Romance and Confession Library #80 c. 1973 which – you guessed it – featured yet another Chatto cover!
The original publication of “A Very Precious Thing” appears to be from Coleccion Celia #169 c. 1964.
Chatto covers were often based on images from the original sources. These covers aren't based on Coleccion Celia #169, but original sources may be identified in future.