Octavia II coolings system

I've had major problems with the cooling system on my September 2005 Octavia 1.9 4*4.

There's a radiator connected to the manifold (a surprise for my local garage) which has been replaced but the cooling system is blocked by some gunge. I did put some blue antifreeze in it by mistake this time last year, but discovered my error and it was flushed out in the Spring. The water's been discoloured and the above radiator was leaking water onto the manifold, which was what eventually prompted a visit to the garage. It thought it was smoke and oil dripping on to the manifold, turns out it was steam and the smell of burning (expensive) anti-freeze. The fans were coming on and staying on and there's been no heat in the system, due to the gunge in the system blocking the radiator under the dash. The engine temperature has been rock solid in the middle of the gauge.

Those are the symptoms and the garage has now replaced the radiator, installed a new thermostat (which was working, but only just) and cleared the gunge they can reach out of the system. There's still no heating, probably due to a third radiator under the dash being blocked by gunge.

Does anyone know what the source of this gunge could be? Is it some sort of corrosion from inside the radiator? Is it a hangover from the wrong anti-freeze?

If I knew the cause I might be able to find something to flush it out.

Cheers