Rangemaster Classic 90 - smell of gas when hobs on?

Hi,
Just over the last couple of days ( weather temp dropped) - when using our gas hob we have been able to smell gas. The hob lights and remains lit - but the smell of gas is clear....How do i fix it? what do i need to do? is it converning - or just linked to colder air or something? Will it cost a lot to fix?
Its a Rangemaster Classic DF 90 - about 1 yr old
Thanks

Z
December 2012