I finally got a new sim card for my old mobile but when I tried to
turn it on the phone was dead. I haven't used it for about 2 years. I
tried leaving it to charge overnight but it still won't power up.

I've tried it without the battery connected but that has no effect. I
can't actually remember whether it could be powered up plugged in,
minus battery, so I can't read too much into that.

I'm hoping it's just a dead battery, but would appreciate any
suggestions. (is there any way to revive a dead battery?)

It's a Bosch 509e, and the battery is described as "Ni-Mh 550 mAh".

The phone itself is practically worthless but it would have been handy
for the car.

I've done fair bit of electronic trouble-shooting in the past though
not on such a small circuit.

Simon