Thank you both very much. I am almost there now - I have 27 out of 28 children sorted and am waiting on two death certificated for unnamed male children to find out who the missing child is. Erik Bloodaxe - I'm fascinated to hear that you went to school with Frederick's great grandson. he would be my third 'half' cousin.Glancare - thanks for the heads up regarding searching through the PRs for Somerset. I had found them but assumed that they were indexed! I should know better by now.
I have just worked my way through all of George Thomas' descendants and I am about to start on Frederick's children, etc. I will look out for (at least) three brothers.