Looks like a plant called Darmera peltata. It has small pink flowers held in dense heads on a long stalk about a month before these leaves appear. These same leaves turn a lovely colour in autumn. It's really an ornamental plant and only creeps through the soil slowly with a thick rhizome so nothing to worry about. It is easily removed by simply digging out the shallow rhizomes if you you want to get rid though.

Rogersia? If you don't want it then you should dig it out now. If it is growing from a rhizome the the parent should be somewhere near, such as a neighbours garden.

Many thanks for the two swift responses.

Fractal is more likely to be correct on the id.

Closely related to Rogersia though :-)

Yep - Fractal was correct and I noticed that the Rogersia leaves were similar.

My wife has now decided that we will NOT dig it all out but keep some.
We were aware of the red leaves last autumn but didn't realise how far the rhizomes had spread over the winter. We suspect that we introduced it unknowingly ourselves by buying an un-named plant at a plant sale last summer. Happy now we know that it isn't a variant of the Giant Hogweed which is rampant in parts of the Tweed valley not too far from us!

Thanks again.