Hello Mike
Do you know if the plastic piping is barrier-type pipe? That can make a difference. The first types of plastic microbore did not have an oxygen barrier in them and allowed the ingress of oxygen. Which might account for the sludge build-up in the system. It's not that old.
Nothing to easily be done about that if it's non-barrier without upheaval and large expense. You asked for some advice....
As an expert, in your shoes, I would find a local EXPERIENCED powerflushing plumber or, ideally a firm that specialises in powerflushing. As in anything, an enthusiasm for the job counts when it comes to the end result.
Here's what we would advise you to do if you called us. Not that we could help you directly. Your miles away from us.
Anyway.... firstly I let you know that some systems kind of reach a 'tipping point'.
There's no way of predicting when it's going to happen; I've lost count of the number of people who wish they'd known enough about their system to realise that something needed to be done as soon as they had suspicions of problems ahead.
Your local plumber is doing the right thing by recommending that the rads are taken out and flushed out with mains pressure and brute force with a rubber mallet. That's got to be done right.
Then the rads go back on and then the primary to the coil and through the boiler are flushed through with mains pressure.
Important: an experienced powerflusher would know not only when to do a mains flush (sometimes called hard-flush), but also when to stop and go back to a softer approach. Flow rates would be monitored and a judgment made on how to proceeed. In this instance I would not add Hyperflush (Kamco) in advance to cope with the ally heat exchanger in your boiler. Normally yes, for a couple of weeks before cleaning. But, if the flow and return are pretty bad then I wouldn't risk encouraging the rust to float around. The answer is to isolate the boiler, flush through the pipework only (after the rads are clean). THEN, dose up with Hyperflush for a week or two.
Whilst drained down I'd put a Magnaclean on if you don't have one already.
The last step is the flush itself. For more info about that you can look at my site at
That's the belt and braces approach to doing right by your heating system.
If the specialists up your way charge less than =A3600 for THAT kind of service you're a lucky man.
And good luck with it. In your situation, give as much info as you can to whoever you choose to employ. That's a real headstart in the hands of a professional.
If I can help you any more do feel free to ask. I don;t know why more people don't install their own systems. It's not exactly rocket science.
Regards, Ian