Great!! Glad my mystery has helped someone else too! Do you remember how many books were in the series? I thought there were three but I can only find The Magnificent 11 which I'm sure was the first, and Showdown at Seabank, which I'm sure was the last. Do you remember if there was another in between?
According to
https://www.ebay.com.au/p/103292729#ProductDetails The Magnificent 11 was published in May 1986, and according to
https://www.allbookstores.com/Showdown-Seabank-The-Dragon-Books/9780583309226 Showdown at Seabank was published May 1987. So I wonder if there would have been time for Tom Tully to write a book in between?
I've checked my local libraries and no-one seems to have it. It looks like it was printed in the mid 80's, which matches my recollection. According to eBay
https://www.ebay.com.au/p/103292729#ProductDetails it went out of print in 1991 so they've probably all been disposed of now, and would also be why no-one seems to know much about them. Thankfully there are still some online so I'll hunt around for them. Seems Showdown at Seabank is much harder to get. No don't y'all go and buy them up!
Thanks all for your help!!
Greg