In reply to Tomkim:
Thanks all for the advice! We had an amazing 3 days up there. Thought I would give a rundown of our itinerary for anybody who is curious as well as summarise the things we found particularly useful or important for any future readers.
The itinerary:
On day one, we took one of the first lifts up to the Midi and walked over to the Pointe Adolphe Rey where we did the Salluard. Grandissimo! From there, it's a short walk to the Combe Maudit where the bivvy spots for the Grand Cap can be found. Day two we attempted L'Echo des Alpages on the Grand Cap. We got about halfway when my partner's foot pain became too much for me to bear (she wanted to carry on bless her) - a combination of performance shoes and blistered heels. Am I still on for the onsight!? For the final day, we plodded back over to the Midi and did the Rebuffat. Mega! Abseiled to the bags and got back to the lift to be congratulated by flip-flopped tourists.
Our key points:
1. Finding/preparing a bivvy. We brought an avalanche probe but forgot to borrow a shovel. It was a bit nerve-racking walking around and having the probe disappear into the snow every few metres. In the end, we adopted a recently vacated bivvy, probing it thoroughly. We were lucky not to need a shovel and will bring one in future!
2. Sleeping surface. We each brought two roll mats. One 'old school' closed cell foam mat on the bottom and one inflatable mat on top. This worked well and was preferable to shivering/shelling out for a good enough inflatable mat.
3. Keeping things dry. We used one of the roll mats with a shiny plastic coating to sit, put gear and so on. We used rain covers for the backpacks to avoid getting them wet in the afternoon snow. Some more sitting/stashing space would have made things even more comfortable.
4. Gas. We didn't bring enough! The stove is not as efficient in colder conditions, even when keeping the gas bottle out of direct contact with the snow. Our desire for hot drinks predictably increased. We also needed to melt snow for water. We figured out we could melt snow during the day whilst out climbing and also mixed in snow with our day's drinking water with variable success. We had brought 50g of gas per person per day. In future we will up this to 75g.
5. Keeping within the law. On our arrival, we saw 7 tents up in the Combe Maudit at midday. We opted to leave the tent up during our second day.
6. Changing approach conditions. The conditions of the glacier evolved quite a lot in two days of warm, sunny weather. The small crevasses we stepped over on the way were significantly wider on our return. Had we started our trip a few days later, it might have been quite complicated to get back. Apparently the approach from Helbronner/Torino was in better condition so we could have gone that way and got the cable car back across to the Midi.
Overall, I think it's a good place for glacier newbies to have a first experience. You are unlikely to be alone bivvying which we found reassuring. The approach is straightforward as long as it is not too late in the season. Just make sure you get to grips with walking in crampons elsewhere before stepping out of the Midi!