Welcome to the forum Dr Evil
I think M80s would be about right for you since you are at the lower end of the 70's. Although the 90's would have been fine if they were not right at the top end (yes, the 99 refers to kg).
Unless you buy new springs I really think you will struggle to get hold of a set of 80's. I really hope you don't give up though. It is a lot of fun.
I would say 50% of your problem is the springs and 50% not being on them enough. The more time you spend the better you will get. If you really clocked up the hours you would be able to work with those springs. Progression probably a lot slower but if you don't want to spend the money on new springs it's probably the only way.
There are some similarities between bocking and trampolining but if you thought bocking would be like having a trampoline under your feet the whole time you were mistaken. It does take a lot of force to get the most out of the springs. You really have to slam down with your heels and give it all you have. The technique can take a while to get down perfect. Most people learn to flip on a trampoline first just to get the feel for flipping but apparently it is quite different with the added weight.
Where abouts are you? Bocking with other people might help you to spend more time on them, and help with technique.
