Ooooohhhh, flecc and Harry that's very interesting, it may do as a top-up at work if necessary. Thanks guys, I've never been to a forum where people are so knowledgeable and helpful before!
Flecc previously mentioned that you could use the Li-ion charger to top up but you would have to be careful not to overcharge (use with a timer?). A bit risky but might tide you over 'till a new discharge charger comes along.