Yep, that's it - worked a treat - thank you. So for example (in case it helps anyone else):
If the individual trunk access code is #22, trunk number you chose is 55 (must be entered in 4 digit format) and the number you want to call is 02920XXXXXX then on the handset (the COS of the hadset needs individual trunk access enabled first) you type
#22005502920XXXXXX
9 for outside line not needed
Thanks