Central Heating removing sludge and changing radiator

Hi,
I have over 25 years old central heating with Glow Worm Boiler. It is working fine and there are two water pumps. I intend cleaning the radiators and pipes and removing sludge from the system.
What treatment should I give to water circulating in the system? How many days should I leave the chemicals to work?
There is some rust showing on some radiators and I would like to change them with the new. They are old British standard threads and now we have metric threads. Any problem it would cause?
House is a large 4 bedroom house with kitchen, garage, dining and bath room. I intend putting power shower in the bathroom.
Your advice would be much appreciated. I shall be visiting my local stores such as B&Q stores for assistance and Graham central heating expert.
Rajinder
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Ch. Rajinder Nijjhar Jatt wrote:

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UK.D-I-Y
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