Holts Radflush / Speedflush. Seriously, just go to Halfords. And read the label...
Do please listen to Robert though - if your hoses and manifolds are not already blocked they will most likely be after you use a chemical flush. I used radflush just before I stripped my engine and despite copious flushing through with a garden hose in both directions, I found the side manifold almost completely blocked with scale and debris when it came apart.
In the course of my rebuild I poked out everything I could from inside the block, likewise the head, new top and side water manifolds, had some custom hoses made, flushed the radiator some more and finally filled up with Penrite classic car coolant. She now runs cool as a cucumber.
Do please listen to Robert though - if your hoses and manifolds are not already blocked they will most likely be after you use a chemical flush. I used radflush just before I stripped my engine and despite copious flushing through with a garden hose in both directions, I found the side manifold almost completely blocked with scale and debris when it came apart.
In the course of my rebuild I poked out everything I could from inside the block, likewise the head, new top and side water manifolds, had some custom hoses made, flushed the radiator some more and finally filled up with Penrite classic car coolant. She now runs cool as a cucumber.