vokera compact 35?

i live in a 2 bedroom semi ..lpg ,gas hob, 7 radiators,i am currently receiving gas 2 monthly ..1200ltre tank .5th march i paid 651.00 to have it filled ,,,now 12th may and need another 1200 litres ...boiler timer is set 5am off 9am back on 7 and off 10pm ...could there be problems with my boiler as i have been experiencing problems with my hot water ..also the boiler cupboard seems 2 be like an oven air in the house seems thick aswell ..am i paranoid but something cant be right that im using that amount off gas in 2 months ...been going on 4 the last 8 months ..any advice would be most helpful ty