Hi Adrian
The Netaheat type boilers have a a fan the pressurises the case. A pressure switch is used to detect the right pressure is reached in the case before that gas valve is opened.
Check your case is fitted properly. There must be no gaps and the seal must be in good condition. (This boiler can kill if a leak occurs)
Then check the fan, does it sound like it is running fast enough. Mine seized up about 3 years ago.
You might have a faulty pressure switch or the plastic hoses leading to it might be leaking.
I have just seen a pressure switch of the right type on ebay at £7 plus delivery the link is http://cgi.ebay.co.uk/POTTERTON-NETAHEAT-1-2-ELECTRONIC-AIR-SWITCH-H-WELL_W0QQitemZ7563889100QQcategoryZ63905QQrdZ1QQcmdZViewItem
You may also have a faultly pcb.
Hope this helps.
Tim
November 2005