Potterton Netaheat 16/22 not firing up?

was working fine yesterday afternoon, yet this morning does not fire up when we turn the heating/water on & does not restart even after we turn the unit off via the dial on the front &/or the main power supply via the wall socket, have left it for 10/20/30 secs & restarted & it fires up, makes the loud click to ignite but does not ignite.

Any ideas???