Thanks for your reply but rest assured the batteries are good as are all the connections.
Just to be sure I also tried a set of new batteries which made no difference.

At 4.8V it won't even power up ........ my parents have a totally un-modified and never tinkered with RU980 which exhibits the same behaviour - down to one bar after a days' use but the battery voltage (measured whilst the backlight is on) is good.

With my added battery (5 ni-mH = ~6V) it still shows 4 bars after 3 days use without charge !