Try Bernies Cafe, Inglesports or Caving Supplies.
All supply Oldham batteries, Enersys Oldham are the manufacturer and will supply you one but they aren't set up for retail sales, you have to send them a cheque and once it clears they will dispatch the goods. Give the Manchester number a call and ask for the Mining Office.
If you like an Oldham type lamp then stick with them, ig you look through the forum their is quite a big thread on the new Oldham DL16 LED lamp and the associated LithiumIon battery to go with it, they are expensive though!