Hi there,
In cultivation G. tinctoria tends to be smaller in all of its parts with more dissected leaves. The main botanical difference is in the flowering stems, which are far more "feathered" in G. manicata, see the link below for a comparison:

http://www.botanicgardens.ie/gspc/targets/news/gunnera.htm

In my experience G. tinctoria is rather less hardy and far less widely grown in the UK that G. manicata. Your pic shows G. manicata.

Ilex - just a word of thanks for this link. I have a Gunnera which I thought was G. manicata, but I was unsure seeing the size they reach in other places.

Looking at the photos of the flowers, it's definitely G. tinctoria. I'll edit my plant list now! :-)

Hi Ilex. Thanks for your info, and for the link, very helpful