I really would like to grow a Gunnera, in my garden.

If its Gunnera manicata it needs a lot of space. Ideally needs to in a bog garden but will grow in dryer sites (leaves may be smaller) although hardy you may need to protect crown from frosts. Although you want moisture you do need it to drain so add plenty of moisture retentive humous in a very large deep whole much bigger than the plant.

It likes a sunny or part shade position in very moist soil, so I'm a bit worried about your 'dry shade' position. Your plant will need a lot of watering even if you create a new environment for it.
As soon as the cold weather comes, break the leaf stems and lay them over the crowns. This should protect the plant, as Drc says.

wet erth will equalwith dry, so trickis to barrier. use wood, your photoabout 10ft across, sp dig down 1mtr and fillwith 4x2wood on4sides to make box. stones inphoto no good asget hot anddry erth.

Definitely needs winter protection as I've lost a couple by not covering them up. A thick layer of straw works well, but don't forget to uncover the crowns in the spring after the last bad frosts.
These are slightly disappointing plants as the ones we see in large gardens are massive plants, but even after three for four years ours are still quite small, although in 'ideal' conditions besides a large pond.
Essex is quite dry isn't it? I would certainly make a good planting hole with lots of organic material to retain moisture, and keep it really well watered during dry periods. It also needs more light than most of us give it! Don't plant in a very shady spot.

Bertie, i htink protectionis key- if they sufer over winterit holdhtemback. isee smallplnts by pond-why, cos clay stop water get to root, even if by water. i had one growto over 10ft in pone yeerby feed and watrewith large leaf over1m wide. it was 2yrold when i put in, with dripfeed water constnt. you are rite about shade, but dircet sun for longtime no good to. ithink a nice big dug hole is good to, so rootsget going fast. myfreend havve Gunnera you can walk under and he only water once a week coshe workand his garden face south! funny things Gunnera.