Does my heating system need flushing?

Just had a leaking radiator replaced. Once removed it was full of black water. The bg engineer said this means I need to powerflush entire system. Other radiators (14 in total) when bled a little show no black water (well two of them had a little discolouration). Just because one leaky radiator has black water does that mean the whole system needs powerflushing? The boiler is a vokera - it has been very reliable. The system was installed new in this house 12 years ago and has been on a bg contract for maintenance and service all that time - how come it is down to me to finance the powerflush?