They are, I suspect, 'avin a larf.
A gas valve, by dint of its name, controls the flow of gas. The gas no doubt heats water that is pumped around your CH radiators. The radiators have water in them - not gas. Even if you had no radiators at all they should be able to guarantee the work of fitting a new gas valve.
They just want to "up-sell" a secondary service and are very very cheeky to make threats of this kind. Is there a symptom that suggests you actually NEED your rads to be flushed or is it a "wise" thing for you to consider to prevent future problems.
Power flushing requires a specialist pump.
Follow this flow chart - use yellow pages and get a quote if you need a power flush...
http://www.powerflushuk.com/diagnostic.htm
Richard Buxton, October 2008
