In reply to Zoe Langford: I would say you'd be pushing it with a 65 but I doubt you'd need above an 85. I have a Lowe Alpine pack that is a 65L but has a floating lid and can expand to take 85L. I find this combination great for longer trips as you can take a big load up to establish a basecamp then if need be shrink the pack down for subsequent smaller trips/ trips when back in blighty.
However fit and comfort is really important especially with heavy loads. If you cant physically carry or it is uncomfortable to carry a heavy load then it makes little sense to get a giant pack. Far better to get a smaller more comfortable pack, im sure your team would rather you took a smaller pack that you could manage than take a monster an either injure yourself or end up moaning a lot. Osprey do smaller packs (as in 65L)that seem to fit this philosophy quite well.
23kg seems quite a lot for 2 weeks- even for sval. If you post a kit list we might be able to both judge the size of rucksack needed better and possibly help lighten your load.