Basically the answer to your question is very big NO.
It is an offence under the Gas Safety ( installations and use ) Regulations 1998. For any persons not suitabley qualified and passed the criteria for CORGI membership. to CCN1, ( Domestic Natural Gas Safety ) To interupt a gas supply and HTR1 ( Fires and wall heaters ) to work on a Gas Fire. Especialy GSiu Reg 8, ( interfearing with a gas supply ). This is punishable by a fine of not more than �5000 or inprisonment. Sorry but your going to have to call out a CORGI gasman to do the job for you & you will probably be looking at �50 per appliance.