looks like the herb sage to me, taste a bit.

Could it be a Hesperis matronalis? (Sweet Rocket)

Or if it keeps coming back, it's comfrey?

id go comfrey too.
specially looking at the root stock on it.
leaves too big for sage and that comes from woody stems rather than more centrally as this looks. Hesperis leaves tend to be less wrinkly and hairy. also not such a woody root. well the ones I ve been digging out aren't.

Thanks, could it be borage?

It could also be green alkanet which I'm sure is related to comfrey & borage, if so it can be quite a nuisance as quite hard to dig up, the roots sometimes being very deep & woody like the one shown. Hope it is sage, that's not such a problem.

My first thought was comfrey but still not quite convinced.
Comfrey sends out a lot of underground runners so dig around where it came from and see if there are any.

Thanks all for replying. After many hours of researching I have to agree with you feverfew and say that I'm fairly sure it's green alkanet.
I did try a leaf and it was pretty bland and it doesn't appear to have any medicinal benefits although apparently is full of nitrogen so I intend to soak it and make a compost tea from it for later in the season.
Thanks again, kind regards John