I have had radiators that did not warm up as much as some of the other in our house, So decided to drain system.After emptying system & then closing all drain points, re-filled system, tried then to fire boiler up (thorn apollo, wall mounted). Pilot light was still alight, but after a bit of clunking & clanging, pilot light went out, now i am unable to keep pilot light lit.Any suggestions to what the problem could be?