TR4 leaking engine rear oil seal

[GERARD_WINN@HP-France-om9.om.hp.com]

     Hi Fellow Triumph Fans,
     
     Does anybody have any experience of changing the rear engine oil seal 
     on a TR4? Apparently this is a fairly common problem on the TRs.
     
     I have the gearbox, clutch and flywheel out of the car. How much more 
     do I need to remove to change the seal? Do I need to go as far as the 
     crankshaft, and if so, is this a real pain with the engine still in 
     the car?
     
     Any guidance would be gratefully appreciated!
     
     Gerard Winn (A Brit living in France)
     1963 TR4