Can I ask, what is the great chess puzzle, and is it free?
It's CT-ART. I think I have 3.0. As far as I can tell, it's head and shoulders above anything similar. It has over 1000 puzzles, ranging from easy to way out of my league. It's not free. I think I paid $26 plus shipping. It's a European program, so I had to wait a while for it to come in, but you're in England, so that shouldn't be a problem.
I have two recommended books for chess improvement that improved my chess playing pretty dramatically. One is
The Amateur Mind by Jeremy Silman, and the other is
Rapid Chess Improvement by Michael de la Maza.
The Amateur Mind has the same material as
How to Reassess Your Chess, also by Silman, but in a much more understandable format. I wouldn't recommend
Reassess Your Chess to anyone, but
The Amateur Mind raised my rating close to 200 points in a few months.
There's also some pretty good advice in
Chess Master at any Age by Rolf Wetzell. It has ideas for study methods, like writing down positions where you made mistakes so you can study them and writing down principles you want to learn on flash cards. Unlike the other two books, Wetzell's focuses on speed chess, too, though the other two will improve your speed chess as well.
The Amateur Mind made it so I was never without some sort of idea on how to proceed in the middle game. I used to get stuck after the opening, scratching my head and thinking, "Well, what do I do now? Attack the king? Are there any tactics available?" No more. Now I always have some idea of how to simply better coordinate my pieces, improve this or that piece's position, or arrange the board to better fit my army.