We have a problem with the Worcester 28 cdi combi - poss ignition lock out due to a fault with the gas valve? The pressure was at 0.7 and have adjusted higher to 1.1, the fan comes on and ignition process begins. No ignition after 10 seconds and boiler shuts down with red central heating light flashing slowly. Money to tight to mention, keen DIYers, is this an expensive job and suitable for mechanically minded please?