A little bit of knowledge can be a dangerous thing. So many well-meaning people have an opinion on anything. So I'll add my two-penneth.
To give an accurate opinion though there a few things I would need to know. they are the questions I ask anyone before agreeing to do a powerflush for them (Based upon eight years of busy powerflushing experience and twelve years installing/trouble-shooting heating systems.)
1. What make, model and age of boiler do you have?
2. Who says there is a serious sludge problem?
3. Where do you live?
4. What size pipework goes into the radiator valves?
5. Is there anything else going on that is bothering you or might be important?
6. How many rads do you have?
And then, over the phone there would be ohter questions that follow on from that. How anyone can pontificate on Exactly what is the right solution for you without talking to you is beyond me.
Some people think that powerflushing is the best thing since sliced bread. Some think it's a big rip-off. Both are right. Not really, but there's no convincing them otherwise.
Personally, I know that powerflushing is great - for some problems.
Other times, it's just one thing available amongst other solutions and only experience can tell which is best.
Occasionally, it can do more damage than good.
So it's horses for courses.
Pricewise, any duffer can get themselves a machine, read the instructions, learn how to take a pump off without spraying water over the place and call themselves a powerflush expert. But, in their heart they know the score and (unless they are a sociopath) will not be able to charge what they would if they really were as good as they claimed to be.
For a professional job you have to pay a professional's price.
Ellis Hill, you don't mention how many rads you have (Q6), but if for example, you had 7 doubles with an old high-water content boiler with a cast-iron heat exchanger (Q1), with 15mm pipework into the rads (Q4) I could give you an estimate of what a professional power flush would cost you.
If you also told me that you live in a hard water area (Q3) so I knew that you needed a de-scale of the heat exchanger too (subject to answer to Q1) and then gave me as much information as possible (Q5) then I could give you a better idea.
After all of that, if it's apparent that nothing else is going on with your system, and that you really do need a power flush then I'd give you a firm price. In this example, £475.00 inc VAT. A magnaclean would be extra but if the job is done well most older boilers do not need them fitted. (And they are only designed to protect the boiler after all.)
I hope that helps.
If you have any questions specific to your situation please PM me and we can talk over the phone. Happy to offer advice whether you are in my area or not.
Best wishes
Ian Pritchard