Partially blocked heater matrix ?

Hi

I posted a couple of weeks ago re my poor heating problem
Steve@Avantgarde suggested back flushing the matrix, I've got the car in with
MB motorcare (MB Indy) in Ellesmere port for various issues and they have confirmed there is restricted flow through the matrix but fairly free when reverse flowing, however no debris has come out even under quite high flow,

I recently changed the coolant and I'm sure from the state of the old stuff it was the original coolant

the heating has been fine at motorway speeds but below 55mph it cools down to barely warm, the engine coolant temp gets up to 90c within 5miles

Is it ok to connect up the matrix in reverse ? and put some de scaler in the coolant

many thanks

Jaz