I've increased the travel on a pair of RS Reba Race 09 from 100mm to 120mm travel.
Now the lock out is not very firm on the initial part of the travel – do I need to increase oil volume to fill the additional internal air space created by the extended fork??

In theory that sounds right, but you could check the online manuals at the SRAM website. If they give different oil volumes in the lockout leg of the fork, then that'll confirm it.

Ive not noticed any issues with the lockout when I changed mine from 100 to 120mm. Perhaps your damper oil was a little low to start. Only way to tell is to get some oil and replace it with the recomended volume and see if it makes any difference? Same volume of oil for all travel options anyway.

I replaced the oil with 123ml as spec'd in the on line manual. They only quote one volume.

Sorry dickie, Ive no idea then? dont know if more -ve air pressure will make any difference? Perhaps take a little pressure out just to see if it does?

OK thanks.
The lock out closed a valve to prevent oil flow thus lock out. So if oil height is the same I've created a 20mm larger air void which I guess I'm compressing prior to more effective lock-out.

I know what your saying Dickie, and the mechanics of what your describing sounds right, but mine dont do it? I run about 15 to 20mm of sag which should put you beyond any small air void in the damper cyclinder.

I think the damper unit sits bellow the level of the oil no matter what length you have them set at.

So it makes no difference as there's not an air gap between the bottom of the damper and the top of the oil.