Some cooler radiators

plumb1_2

If it is a old Yorkshire type manifold I would cut it out and replace with tees and reducers.

looking_forward

I am not sure what a "Yorkshire" is but here are the 2 in all their glory , my red marks indicate the redundant cut pipes from the old radiator that was in the kitchen.

 

Bendy_House

Nice work!

A controlled blast from the mains is good at removing airlocks from gravity systems too. The flow from just a loft tank has such low pressure behind it, that a raised bit of pipe with a bubble of air can easily hold the flow back. Pushing that bubble out of the way in reverse is often enough.

It does sound as tho' you do also have some sludge at least, but probably just concentrated in a few points - low areas of pipes, for example. So, a filter plus a strong dose of X800 or similar, and see what you collect.

looking_forward

I gave the system a dose of X400 about 3 weeks before I initially drained. I will give the X800 a go but have heard some scary things about how powerful  it is 

plumb1_2

Looking at those manifolds (Yorkshire type) there have been a few small leaks, but self sealed
using 800 is a bit strong for a diy clean as if you leave it sitting there you could possibly end up with small leaks .plus you will need to flush with clean water a couple of times to remove all the cleaner.
I would use 800 , for microbore but I have a kamco machine and I link it up with a magnacleanse. Combined with a rad hammer. Constantly switching flow/reverse.
Firstly cleaning the whole system then each individual rad, then the whole system again with clean water. And testing the ph .Average time 6/8hrs done properly.