Thanks all for your help
I had a look again at the circuit last night and it's pretty complicated. Quickly applying a 1.5V battery across the LED in question make it glow but it also makes about 5 others glow faintly too so I assume they are connected in series.
I took a chance anyway and took it off the board and soldered in the red one and it seems to work just fine. Doesn't look too bright or too faint so I'm hoping the voltage difference is kind of absorbed in the other LED's and the resistors or the other LED's won't fail.
Time will tell I guess. The original LED came of the board okay so I can replace it again if the red one gives up.
Yes, They are small things to solder! I have a variable temp soldering station with a very small tip and 0.5mm soldering wire but it's still kind of fiddly!
Thanks again
Craig.
On the circuit board they and their associated resistors would have been in parallel. When the 1.5 volt cell was connected across just one of them, it would have glowed, not too brightly because of the internal resistance of the cell, but there would have been a path for current the wrong way through that LEDs resistor, and a series path the right way through all the others which would form a parallel network and be working at a much reduced current.
A lot of places give specs for LEDs that leave you a bit short on information - sometimes the current quoted is the max, above which the smoke comes out, other times it is a current that causes a given intensity of glow. In this case, if the LED is glowing in a satisfactory manner, and its resistor isn't cooking, then the jobs a good 'un. There was always a good chance that the resistance required to limit the current to one LED and result in 3 and a bit volts would be the right-ish value to limit a slightly higher current to the slightly lower voltage that the new LED wanted, the only uncertainty being the power dissipation while doing it.