Colin you will have to try and forget about what you have been doing(harder than you think)and start from the beginning from laying the line out straight behind you then gradually work your way through it a little at a time. I use to do the overhead thump like a lot of people do got about 60-80 yards, by just laying the line out straight behind you like he suggested
my distance went up to about 120-130 yards just after about 12 casts. I would say without a doubt that this book would be the best surfcasting book you will ever buy.