Been out cutting some slabs today so thought I'd report back...
Definitely seems that the blade direction was the issue...took off the dust guard and rotated the grinder, and it now cuts through slabs much easier, with the blade pulling the tool forwards into the material as it should do, so not really having to apply any pressure. Although I do now have the problem with dust (previously the shroud and hoover did a great job of removing all dust), and have rigged up a dodgy water contraption to keep water on the cut.
However, I do have some issues with this:
It is much harder trying to cut a straight line without the dust shroud, as I have no marker to follow, and because the tool is now pulling into the material. I have also found that I now get lots of little flaking along the cut line on the surface of the slab, whereas with the dust guard on (and blade cutting in opposite direction) it was a perfect clean cut. Much easier to keep tool steady and cut a constant depth with the dust guard.
So I have 2 options:
1. Go back to the dust guard, cut nice straight and clean lines, no dust but get through lots of blades (will probably cost me £200 in new blades)
or
2. Take dust guard off, cut in correct direction but have slightly wobbly lines and lightly chipped surface along the cut. And the problem of how to deal with the dust (maybe rig up a better water solution). But I will probably get through all remaining cuts in 1 or 2 more blades
Not sure what to do, will probably take a look at Youtube and see if there is any advice on cutting better lines with an angle grinder