I doubt the gulls are pecking to get into the tunnel for a start off they don't eat flies/bugs. They feed on worms, small molluscs, small fish etc. their beaks are not designed to enable them to eat any sort of insect! No idea why they are attacking your polytunnel but it isn't for food!

Do you eat or keep food in your work area? Unfortunately, if you have, even if you stop now, it may take some months before the gulls give up.

could it be because they are attacking their own reflection thinking its a mate just a thought good luck.

We don't keep any food, it is a small covered area about 6feet wide by about 4 feet deep and covered in a clear plastic and open to the front. I have been working on the plot when the gulls have been pecking on my neighbours plastic greenhouse. It is very strange as we couldn't think of any other reason why they would want to peck through the plastic, other than the bugs.

Any ideas to stop them landing on the new poly tunnel?

If you look at any gulls bill you will see it is not designed to eat insects. There has to be another reason why they are pecking at your neighbour's plastic greenhouse. Unfortunately there is little that will deter a determined gull! Perhaps you could ask your local council for advice.