Hello Jenny. I have a very similar problem. My poly tunnel is at the top of an escarpment in Kent. It really blows. Great views, but jolly windy at times. My tunnel was a gift, but not meant for a windy site. I had a local blacksmith make some fearsome stakes/ hooks to hammer into the ground over the base frame.These have held up, however the cover is taking the strain. Researching this, I found the advice was to make a wooden base frame and securely tie in to each of the base poles. The wind catches under the plastic and acts like a kite. The base planks are are fully joined together to make a rigid support. The cover of the green house can then be stapled or attached with tacks to the base frame. This offers weight to the bottom of the greenhouse but also prevents the cover from lifting. I am about to do just this. I am happy to email pics if this helps. Hopefully my tunnel is still there after todays wind. Goodluck.