A lot of what you need is in the attached.
http://www.pprune.org/questions/3425...-pilot-ftl.htm
Essentially if you fly as a co-pilot (or safety pilot is a better description) of an aircraft certified as single pilot then you can't log anything. As you will probably know to get your ATPL amongst other things you need 500 hours in a multi crew aircraft. Therein lies the problem for you. On one hand you will gather valuable time and experience sitting in a commercial aircraft flying IR and airways but on the other hand your logbook will have nothing to show for it.
The only way around this if you hold a JAA licence is to (a) have a type rating on the aircraft and (b) ensure that the AOC certifies the aircraft as needing two crew members.
On a related matter it is not uncommon for insurance companies to require a safety pilot even though the aircraft is certified by the AOC and manufacturer as single pilot. This occurs quite a lot when flying VIP's and medivac. Unfortunately you can't log any of this time.