Fixed it!
And as this is relvant to another thread where I was trying to flash an ex Sky modem with DGTeam firmware, and failing, I'll add an update there too.
So :
I downloaded the DG834 recovery tool, and the most recent official Netgear firmware from their site. Ran the recovery software, exncountered the same problem I'd had trying to update the Sky modem - the recovery tool window becomes corrupted, and nothing happens.
The recovery software gets as far as asking you which network adaptor to use (I've only got the one) then the window gets partly overwritten and pressing buttons makes the display change but nothing else happens.
A bit of googling found various solutions to this, one of which was to install a 2nd network adaptor. As I had one handy, and it was a Netgear adaptor, I've just tried that. And it worked!
I added a 2nd network adaptor (Netgear PCI gigabit one), connected the dead Netgear DG834 to that, and ran the recovery program.
I put the Netgear into recovery mode by holding in the reset button whilst plugging the power lead in, making the lights on the front flash alternately.
Recovery program showed me both network adaptors, I chose the newly installed Netgear one, and it correctly displayed firmware versions etc, then flashed the modem, and it's all working again!
Might try reflashing with the DGTeam firmware a bit later, after I've checked I downloaded the correct version!
Ian