I replaced ours myself, as a present for my OH - who hated our dark bedroom!
Once I'd taken out the old Heki it was easy enough to put a stool under the hole in the roof and get up far enough to scrape off the rest of the mastic - easier than trying to lay on the roof, I would have thought.
TIP (Which I was given at our local caravan dealer) - use those wooden spatulas that you cook with - but 'sharpen' the edge, so that it's strong enough to get under the mastic, but soft enough not to damage the roof. Soften any remaining mastic with white spirit, making sure you get every last little bit off. Use the strip mastic to replace the old, making sure that it's wide enough (I think it comes in several widths). It's not a difficult job at all and has made a real difference to our bedroom.