After finally discovering just how easy this job was I just had to post details for others, and save yourself 
£100 in Lexus labour fees! Thanks to is200 Newbie for his excellent tutorial
and to normski2 for pointing out some time ago on a distant thread just how easy it was to do this!
Right here goes, hope this helps somebody, and if my post needs improving please let me know......
1. Pull off wing mirror glass panel - simply by pushing it back at the top and out at the bottom, then spreading your fingers across to distribute the load, pull the mirror away from its housing (and dont be afraid to apply some force!). It helps to open the door window and lay a cloth on it to rest the mirror on while you do the job.
2. Remove 3 screws, which releases the back of the wing mirror.
3. Remove 2 screws securing the indicator lamp in place.
4. Remove the power lead to the lamp, by pressing on the side of the connector to release it, and pull it off.
5. Lift off the indicator lamp, then connect new lamp (remembering to fit the two screws, which I didn't!)
6. Clean everything, ensuring the back housing fits tightly, which I lined up at the tricky bit under the motor.
7. Put three screws back to secure back housing and reinstate mirror.
