Went back to Grafton today and had another go. This time I used water instead of air; about eight litres in each hull. I lifted the front of the boat and straight away water started dripping from the transoms onto the concrete floor of the shed.

The water is definitely leaking from the filled bung holes. Funny how we couldn't get any soap bubbles there last week. I will start the repairs next week.

Thanks to everyone for their advise.