Thanks to my friend Andy Chillingworth, I bought the 90º Constrained cube. Only single 90º turns of any face are possible and once it's gone in one direction then it must go back the other way. This turns a straightforward 3x3 Rubik cube into a really really hard puzzle.
How do I go about it? I don't want to give too much away inadvertently so click the show button if you want to see my broad outline:
Check your solved cube for how many are clock vs anticlock turning faces
Aim to have the same number in the final solve
Solve bottom face edges
Solve equator edges - using the Edge piece series
Orient top face edges using F U R U' R' F' or it's mirror (standard beginners method)
Position top face edges - move correct edge to R, then use above alg, rotate clockwise and repeat. Then turn cube back and repeat x2 - May need to do variants of this - aim is to move edges around.
Position corners using Corner piece series or EPSx3 - will need lots of setup moves
Orient corners using EPSx2 followed by a turn of U and undo the EPSx2
