I suggest you go and buy a specialist rose food, such as Toprose, and apply that to the roses. If your soil is particularly alkaline, you can also give the roses a shot of Sequestrene iron tonic as well as the rose food.
For the camellia, you say you've already used Sequestrene, so that should help, but suggest you give the plant a feed with something like Growmore raked in at the base as well. Note that Sequestrene is simply an iron tonic, it contains no other nutrients for the plants, such as nitrogen, etc, so that needs to be supplied separately. There are specialist acid feeds available (Miracle Gro make one for Azaleas and Camellias) but they're expensive and as long as you use the Sequestrene, an ordinary feed with Growmore will be fine.

could the sudden hot weather caused a drought for them ?

Thanks Bamboo and Pamq for your answers and suggestions. I have applied the Miracle Gro to the Camellia as I already had this and "fed" the rose and will continue to water both. I was wondering whether the hot spell had affected the rose in particular as its condition was very rapid, from glossy green leaves to those awful yellow ones a few days later. There are still loads of buds on the rose, so hopefully I will get some good blooms. Thanks again