Why does my Potterton Lynx boiler not work? Please help a single mum!!?

Recently moved into new house and told water and heating working fine (although due to weather I haven't had the heating on yet). Hot water has been perfect until yesterday, when a friend replaced 1 radiator. Basically, when i turned my 'flow' & 'return' taps back on underneath the bolier - since then i have had no hot water and boiler doesn't seem to fire up properly. The DEMAND light comes on, then a couple of seconds later the LOW FLAME then HIGH FLAME lights come on in sequence for 1/2 second each and water gets tepid, then lights go off and water goes cold again.

I hope i don't need to buy a new boiler as this one, though old, should have life left in it. I have no idea about plumbing and as a single mum can't afford a plumber right now.