Hi tricky.
I'm a gas safe registered engineer who is new to the forum.
I would charge around £325 to powerflush your heating system unless it is 8mm or 10mm microbore pipe work. I use the kamco powerflushing machine, which is a great piece of kit. I only use quality kamco cleaning chemicals and crystals. Upon completion 2 bottles of fernox F1 inhibitor would be added to system. To install a magnetic system filter would cost an extra £90. This is recommended as it keeps on protecting your system for years to come with no need to buy any replacement filters.
Cheers Kev (direct gas & heating services) 07789 405751