Looks like a fungal infection called "Rust". Key indicator to this is that the leaves turn reddish on one side. You might try applying by spray a fungicide to get it under control.

If its rust, you should be able to see orange circles underneath the leaves, otherwise, the way the plant looks is not typical of rust infection. The reddening is likely shortage of water. If it turns out to be rust, fuchsias often react badly to being sprayed with fungicide (the few that are available in the UK) treatments, see link below

https://www.rhs.org.uk/advice/profile?PID=163

Thanks Loosestrife2 & Bamboo for your help very much appreciated.