Anyone know what this plant is? and the caterpillars on it?

There is an ap available where you can send a pic of your plant and they can usually identify it!

I used to have that on my phone, but can't find it any more. Will try searching for it again.

Looks like cabbage white to me ,not sure of the plant.

Your plant could be a Verbascum

I think it is a Verbascum olympicum - lucky you !

Not sure about the caterpillar

or a perennial cornflower (centaura montana)

One appeared in my garden, I thought it was a weed but a friend assures me it is not. I haven't noticed any caterpillars on it although it is in rather an obscure spot so I haven't examined it closely.

Or are they Black Swallowtail caterpillars?

Yes it is a Verbascum or mullein. The caterpillars will be of the mullein moth, black yellow and white - pretty. They can strip the plant so it would be good idea to pick most of them off.

I think the plant is a yellow mullien but I may be wrong. Have a look at

www.rhs.org.uk/Plants/177991/i-Verbascum-epixanthinum-i/Details

if it's hairy it could be cabbage white

black swallowtail is very rare so an optimistic thought!

The plant looks like a mullein/verbascum, so the caterpillar could be a mullein moth.
www.gardenersworld.com/how-to/solve-problems/mullein-moth/

Ignore me, no wonder I was not invited to present Springwatch.

The plant is definitely a verbascum, and the caterpillar will become a large white butterfly. Lovely yellow flower on the verbascum.

Mullein moth and cabbage white caterpillars have very similar coloration. But the cabbage white ones have little hairs.
www.gardenersworld.com/how-to/solve-problems/cabbage-white-caterpillars/

Two years ago the cabbage whites picked off an entire tray of rocket I'd been lovingly cultivating.

We had these a couple of years ago.....both the plant and the caterpillars. The Plant we got rid of as it got too big for the bed...the caterpillars do still crop up on our small bush buddleias and will devour them if given the chance. I manually remove them and throw them over the hedge!

Definitely the mullein moth caterpillar - cucullia verbasci (so it says in my moth book!)

Thank you all. You are so knowledgeable! The caterpillars will be picked off as and when they are seen. I'm away for the next month, so I might miss the glorious flowering this year, but will look forward to it next year. Thanks again!

I love random arrivals and mullein is a favourite. Send it to me if you don't want it but no caterpillars.