Thanks for the comments so far. To answer Swbjackson's query, originally one leg of the 2 pipe system fed the rad's at the front of the house and the second leg fed the rad's at the rear of the house. To 'improve' the flow I connected the ends of both legs to form a ring and then from there ran...
My house is approx 10 years old and has a central heating system using 10mm plastic piping. For the last 5 years the system has had to be regularly power flushed once or twice a year due to 'magnetite' build up.
It was very rare that all rads were up to heat. To improve the circulation I...