Q for those familiar with MagnaCleanse flush

[clueless78]

Doesn't mean it wasn't in good condition, some of the older gravity systems I support are the cleanest systems I work on.
Yes, you can use the bleed screw on the rads to sample the water from the rads and use the drain to drain some water from the system itself too, do both, just go carefully using the drain valve as it is under pressure if you system is sealed, you don't need much. Check/top up the boiler pressure when done.

I'm fine with getting the water from the bleed screw but wasn't sure how to get some from the drain cock, the youtube videos I'd seen had the drain cock buit into the valve whereas mine are separate. Would be grateful for any pointers... pic attached.

It was a combi boiler replaced with a combi boiler, not a gravity fed system, not sure if they are naturally 'dirtier'.