Sorted - combination of other forums and a bit of lateral thinking.
Firstly - USE THE GENUINE LANDROVER PART !!! I got some aftermarket ones from Paddock and while they look ok, when you compare them to genuine landrover ones (which I managed to get today before they shut) the aftermarket ones have a slightly greater flare on the skirt of the dust seal which effectively makes it just that little bit bigger and hence much harder to fit evenly into the caliper.
Second - even the genuine ones aren't that easy to fit so after a bit of head scratching here's what I did and it worked:
1. Fit the inner fluid seal as normal;
2. Insert the piston (I was fitting new ones which probably made it a bit easier) and push it in until there's only about half an inch protruding out of the caliper;
3. Fit the wiper seal into the dust cover & smear with brake fluid;
4. Push the wiper seal/dust cover assembly over the piston and down to the caliper face just with your fingers, making sure it's flat;
5. Lay your hands on a small length of metal pipe with an inside diameter of around 42mm (which just slides over the piston) - I cut mine to a length of around threequarters of an inch and make sure the cut is square. Remove any burrs;
6. Put the short bit of pipe over the piston so it rests on the dust cover you've just fitted - the top of the pipe should be a little proud of the top of the piston;
7. With the caliper on its side, put a length of inch (or similar) wooden lath through the caliper, resting on the top of the pipe;
8. With a hand on either end of the wood and using even pressure, push down on the pipe and this will seat the dust seal evenly into the caliper. You could use a G-clamp instead on hand pressure I suppose.
The genuine units went in first time, every time. I experimented on the final piston using the technique above but with the aftermarket ones and wrecked the dust seal each time.
No doubt others will have their own 'pet' methods but this one definately worked for me.