I never had any probs, even with the red on switch :dunno:There's not a lot of space to play with inside the switch casings to mount the new LED with resistor. My advice is to attach the LEDs and resistors as shown in the configurations below. There are cavities in the casing where the resistor will not obstruct the mechanism or otherwise get in the way.
New LED and resistor soldered in place:-
After following the excellent guides above I had a crack at this last night when I got back from the bar - looks like a few pints of carling does help the grey matter!
I found soldering the resistors into place was extremely difficult without obstructing the operation of the switches, especially if replacing the 'on' led with red.
If you turn the switch over, cut out a small V in the plastic housing and mounting the resistors on this side there are also almost pre made holes to feed through the legs to the LED's. This also sped up the process on subsequent switches.
Edit: Maybe I have smaller resistors as mark said :dunno: