Brrrrrrrrrr!!! Help!

[Street]

This was my effort
I went with Fernox central heating cleaner. Pulled loads of scale / build up out into the bucket and had a minor improvement.
Wouldn't bother again though to be honest.

[Pondrew]

That's what I'm going to do mate. Bucket of mixed rad cleaner, bilge pump and then into empty bucket backwards and forwards.

You could hire a manual domestic heating flushing/ pressure testing pump. That would give you the ability to pressurise the whole matrix and then fire it out under pressure to get any crud moving. May need multiple attempts and you would need something to cap one pipe temporarily that you can take off easily.
Just don't pump the pressure too high if you try this, as (I presume) the matrix will be aluminium and could burst.

[x-driver]

This part is a common problem but the E85 doesn't seem to have one. You'd find it easily in the ETK diagram but there isn't any. It's however interesting that it's being sold for E85 but they clearly don't know they are useless to the buyer.

[caius]

The matrix is quite lightweight for its size, so it likely is aluminium. Mine had a leak last year in the 2.5i, so I ended up taking the whole dashboard out to swap it for a secondhand part. The whole heater lump needs to come out and then come apart to get the matrix out, that way lies madness. (Also needs the Air Con de/regassing to do it.)