Did you make sure the flowers were pollinated? You will get less pollinating insects in a polytunnel and may need to do by hand with a child's paintbrush.

You aren't alone, Tynwold. Here, where with our (sometimes) much warmer, drier weather, we still have problems. I planted aubergines for three summers - the first lot from sown seed, the next 2 lots with seedlings bought on the market. In all that time I have got precisely 2 slightly undersized aubergines. Here they also sell what they call "piante innesti" which roughly translates as "grafted plants". They cost a bit, and are about 60cms high, and our friends just buy a couple of those to provide them with dozens of fruits throughout the season. I think they must be a really difficult crop, and need to be started early on in a well-heated greenhouse. I for one shan't be bothering again!

This is not exactly to answer your question but to say a few things from my own experience. I do grow aubergines, tomatoes, peppers and cucumbers. Although I always grow them in the same spot with not much sun or access to insects for the pollination, my success differs from year to year. Last year I had so many aubergines I didn't know what to do with them. In my opinion success and failure rely more on mother nature and less in human effort. I do not know how many people share my view but my father who was a farmer used to say that if it's going to be a bad year for a particular crop it will fail what ever you do. If on the other hand is going to be a good year, success is guaranteed even if you do nothing.

Thanks for the info on Aubergine, much appreciated.