Thanks very much for your suggestion and sorry it's taken so long to reply (family issues). I'll give that a go. While all that was happening I won an eBay auction I'd forgotten I'd bid on for an mz r55. My idea was to use the dry battery case from that. Oops, the contacts are in the right place but the connecting screw is a different size. One good thing though; the original Sony battery that came with the r55 will work the r900. Go figure.
Just bought an old MZ R900 on eBay, it was listed as not having a battery so I bought a gumstick but when it turned up the contacts look pretty corroded and it won't charge. Thought I could try the add on ones unless anyone can suggest a different solution?