Alkaline manganese batteries, a recent innovation, contain much more charge than the equivalent size dry battery. They produce the same voltage and so are direct replacements for dry cells. The Ever Ready MN1500, which is standard pencell size, contains 1800 mAhr, according to a telephone conversation with Ever Ready. This should give 6 hours light with a standard 0.3 amp bulb.
I carry a Pifco single cell torch containing an MN1500 as emergency light. It takes up hardly any room in my helmet. It gives a rather feeble light, of course, but sufficient. The torch is preserved by coating the battery and contacts with vaseline. (from Boots: good for NiFe cells too.) The whole torch is then wrapped in polythene. It's possible to check the torch works without disturbing this. In use the torch is pushed into the loop at one end of my helmet strap.
Prices are about 2/9d for the torch and the same again for the battery from places like Currys. The shelf life of the cell is about 18 months and so it should be replaced at this interval.