I have just taken part of a studded wall out for modernization reasons, but found a gas pipe right there in the middle of the aperture. The easy way round this would be to use a 'flexi hose' is this ok to do so?
Most likely that's a copper or iron natural gas pipe. Did that pipe feed a wall heater at some point?
You'll need a licensed gas fitter/plumber to re-route or remove it.
I wouldn't attempt to do that work yourself. If you did and there was ever a problem (leak) or fire that was later traced back to any change you did, then you could be up for some insurance problems.
As Tester101 mentioned, a flexible hose is not suitable for concealing in the wall.
Similar to plumbing that is in the way, the pipe needs to be re-routed and moved. Depending on your location, this might be something that is permitted by the home owner or it might require a licensed technician to complete.
Unlike water where a small leak is unlikely to kill anyone, an incorrectly plumbed gas line can be deadly and as a result, unless you have experience working with gas lines, it is best and safest to call someone in to complete this for you.