Thanks for the welcomes and replies folks, sorry for some reason notifications were going into my spam.
@gunwharfman - the issue with a rucksack is having no back to carry it on once it is emptied of the carrier it will contain, if that makes sense?!
However, I have picked up two solutions:
Firstly, an Osprey Ultralite Stuffpack Rucksack which squishes down tiny enough to fit into the pocket of the back carrier itself. This is probably the best solution.
Secondly, I also picked up a 12L Haglofs Technical Waist Pack (from eBay, I think they must be discontinued) which fits the carrier neatly, with room to spare!