Thanks for the replies folks.
Update - the replacement bulbs arrived yesterday and the first test run was a complete sucess. The illumination pattern and brightness of my headlamp were much improved, with a less expensive bulb! On the unlit section of my journey, the roadway was lit clearly like never before
I now have two operational headlights with much improved performance and 9 spare bulbs!
For those interested, the light I use is a Cateye HL-1600G rechargeable and the bulb substitution is 6v 4.2 watt in place of the 6v 2.4 watt type with PX13.5S base. The compromise is run time which will be reduced to approx 60% of the original for a given set of batteries. As mentioned in the original post, I replaced the original NiCad batteries when they wore out with high capacity NiMH types, which puts me roughly back where I started.
Cheers,
Ed