Bit late for original post, but this may help others.
I had the same issue: how to remove pistons from calipers off the vehicle. On buying new pads at my local LR spares supplier, I asked if they did a removal tool: No. They suggested using compressed air.
But once you get the first piston out with pressure, you have to seal the hole left, and the fluid ports for either piston removal.
Laser make a piston removal kit, but it appears only for motorcycles (the adverts now keeping popping up here, since I was looking: £110!) but they are not big enough for Landy pistons.
So I went to my friendly Landy man to borrow his removal tool, he doesn't use one. He wacked the caliper on his anvil a few times and shocked them forward, level with the flanges, and suggested pulling with mole grips the remaining distance!
I did the same with the other three caliper halves, but using a block of wood on the ground, and then with small sections of timber at the end flanges, so they could move further forward, and I managed to get all 8 out!
Easy peasy, and I am still embarrassed that I never thought of it.
Matching new pistons to the caliper repair kits is not quite so easy, see my post under projects