Worcester 24i Junior - Only stays on for 5 Secs?

Further to my post about the melted water flow switch filter, the boiler fires up but only stays on for 5 seconds before turning off now, for both water and central heating demand. No flashing lights on front or anything. If I disconnect the Air Flow switch and blow into the red tube it will then fire up again for only for 5 seconds. Really flummaxed. Ta!