I found your update on putting up your polytunnel very interesting as I shall shortly be doing the same. Why did you put bonfire ash on one of your beds in the tunnel? Was it done for a particular crop or for some other reason.
Actually it was simply because the bonfire had just finished! I like to spread it hot before all the charcoal turns to ash, and that bed was available to spread it on, so it will be interesting to see if there might be any difference in growth between that bed and the other middle bed with same mulch and no ash.