In the unlikely event anyone is interested….
Success! On the second attempt. Despite cleaning and looking very closely, I couldn’t be sure where they were leaking. I assumed the join between sole and upper. I carefully applied Aquasure all the way round with a cocktail stick. I thought I’d done a neatish job. I was gutted when I tested them next day in a bucket of water and could immediately feel water seeping in. I just couldn’t see any bubbles coming from anywhere!
Drastic steps. I put my Titan garage vacuum on blow, put the nozzle in the top of the offending welly and made a loose seal around it with my hand. With the welly in a bucket of water. I could see bubbles coming out of a tiny ( but obviously deep) hole in the thickest part of the sole near the heel.
I poked as much glue as I could in the hole then made a little dam around it and sealed it over the top with a little glue ‘scab’. It seems really strong and is definitely waterproof now!
The vacuum also did a great job of drying the inside of the boot. I gave them a clean with some proper rubber dressing and my 15 year old wellies are as good as new.
Little victories!