Them gate valves are renowned for seizing up, best option is to change them for some quarter turn valves. If you feel confident in drainin it all down you can do it yourself. Where bouts in London are you.
You are not allowed to use a flexible hose to connect up a hob, it must be in rigid pipework!!!!!!!
Pm me on here if you need help it advice as I'm gas safe registered
How soon are you looking to get this done as it will be easier getting a gas engineer out to have a look at this and give you some advice.
What's your location as someone on here may be able to help.
Will you be doing the work yourself or getting a plumber in, it would be easier to give better advice looking at the job in person. Put your location up on here and someone may be able to help you.