Having watched a youtube video I went ahead and pried off the front screen. It is glued firmly in place but if you work round the edge methodically, it eventually comes off although some force is needed. Take care at the bottom RH corner where the ribbon cable lives.

Next, unscrew the circuit board and lift it out. Unplug the battery from the circuit board. The battery is stuck to the case but can be easily pried out.

I cut the wires from battery to speaker then soldered the speaker wires to the new battery. The new battery came with the necessary plug and wires. The old battery is 700mAh but the new one is rated as 600 mAh.

After this I reassembled. The Garmin started up fine but... no sound. I opened it up again and found the plug was slightly loose. Reassembled and all was good with those familiar beeps. To replace the screen I put a little bit of 'No More Nails' DIY glue round the edge and put a weight on the thing till the glue set.
It will be interesting to see how the battery life is from now on.