how do I fixmy ariston d I a 20 mffi combo boiler?

we drained our central heating system today to move some old pipework. When we pressurized the system it worked fine. After a while we had to bleed a few of the radiators and top up the system again. at this point the boiler was still working fine, but suddenly switched itself off. I had to use the red warninkg light to reignite the boiler and this worked each and every time I had to do it over the next 35 minutes and 20 times. The boiler would cut out after 20 seconds or so and I would have to repeat the whole thing all over again. I also used the thermostat to try and keep the system on but to no avail. Help!