can anybody tell me what this is?

looks like a hawkmoth caterpillar

certainly is and possibley an elephant hawkmoth.

yup elephant hawk moth feeds on willow herb and ladys bedstraw, lucky you theyre wonderful things if it was wandring about its probably looking for a new food source which means its an "old" one feed it willow herbin a cage made of old net curtains and you may get the chrysalis stage soon keep it safe till next year and it wil reward your efforts with a beautiful moth to release back to the wild

It has a voracious appetite for fuchsias if you have any. I mean protect them not feed them to it.

Do sacrifice a few fuchsia branches for the sake of this one caterpillar, unless you have rosebay to hand. It won't be long before it pupates!