DIY it, heres a few tips:
1) Close the valves either side of the rads!!
2) Make a wooden wedge/s out of a bit of softwood floorboard. Use a joiners axe to shape it to valve hole, old valves often leak you can’t tell before hand because the system is sealed, the wooden wedge quickly swells and blocks said leak from valve leaking on to the floor while you are in the back garden flushing the rads!
3) Drain as much out of each rad as necessary to carry and flip (you don’t need to drain all of the water out till you get to the garden), 2 people blocking the valve holes with thumbs then lift and quickly flip no more drips!!
4) Hose out the rad till water runs clear can take a while!
I’ve resurrected 70’s systems that have never been serviced this way, that have then gone to work for over 10+ years.
Have fun 😉