Wiggy
Is that a 737 procedure only Tony? I only ask because it's certainly not the approved manner of handling engine failures on some other types, and in any event ignoring the SID could bring you a world of pain departing many airports.
It certainly is on the types that I have flown for real and in the simulator (B727,B737,B747,B767 BAC 1-11 and L1011). All the old performance A. Type A and Type B ICAO airport charts only accurately survey the splay and or designated emergency turn. If you look at any of the SID's for LHR you will see a minimum reqd climb gradient of around 800 fpm. Any transport aircraft at max regulated take off weight will probably not achieve anyway near that with an EFATO.
The Max regulated take off wieght is computed assuming you are going to carry out the procedure that you are trained to do. If you listen to the Thompson video you will hear that they went to Wallasea on an assigned heading which is designated in the emergency turn for Manchester, not the SID. ATC will expect you to carry out your procedure once the mayday call is made.
There is a great publication from Airbus called Getting to Grips with Performance. Its good reading, revision for all pilots.