Baxi Bahama 100?

My Baxi Bahama 100 made a crunching noise soon after it started. It then stopped. I noticed the water pressure was low, so I topped it up to 1.5 bar. It repeated the same again. Then it started again but within seconds the pressure went up to 3 bar. I shut the boiler down as It started to leak from the heat exchanger. I fit a new heat exchanger and pressurised the system to 1.5 bar. No leaks now but when I try to start it, the fan works but it does not even try to light!! The "boiler li" light flickers bt the boiler is not alight. Any ideas? What sequence is the boiler suposed to go through before it ignites?