Sime Format 100c Making tapping noise then the boiler cuts off?

My boiler has starting making a strange noise for central heating only?

It was a loud tapping noise, i turned the boiler off an all the pressure had gone so i put the pressure back to 1 bar and bleed the radiators, a lot of air was in them,

But now i notice that the boiler will start up for a while the pilot light will light up but then after about 20 seconds it cuts its self out?
The pressure has stayed the same now at one bar.

Any ideas on this? and how much would it cost to fix?

Paul Aldred
September 2007