How many ovens does your Aga have?
If it's got 2, the roasting oven is at the top right, and the simmering oven below it.
If it's got 4, the roasting oven is at the top right, and the simmering oven at the top left.
The boiling plate is on the left, and the simmering plate on the right.
Veg - including potatoes - bring them to the boil on the boiling plate, let them boil for 5 mins max (spuds 10), drain the water, bang the lid on and put them in the simmering oven. They steam. On their own.
In the roasting oven, if you have the grid shelf on the floor of the oven, that position is about gas mark 6 - fine for pastry, shelf half-way up and it's about gas 9 - so fine for Yorkshire puddings.
If you want to cook anything like a Victoria Sponge, and you only have a 2 oven, you'll need the cold plain shelf - bit like a baking sheet. You put the cake tin on the grid shelf on the floor of the roasting oven, and the cold plain shelf about half-way up. It's ok for cakes that take less than an hour - longer than that, you start in the roasting oven, and then transfer the cake, placing the tin on the now 'hot' plain shelf and put it in the simmering oven.
The boned stuffed chicken will be fine in the roasting oven.
One of my favourite Aga recipes is a boned rolled shoulder of pork.
Score the skin about 5mm apart for crackling.
Put the pork in a pot, add a bay leaf and a few peppercorns, and a carrot and a quartered onion if you wish. Half fill with either water or cider, put on the boiling plate, bring to the boil, put the lid on and bung it in the simmering oven, for about 6-8 hours or overnight.
Remove the pork from the stock, and keep warm. Line a sieve with 2 sheets of kitchen paper, and dampen by running through with cold water. Strain the stock through this and you'll get lovely clear stock and all the grease gets left behind on the paper. Boil the stock down. Put the pork in the roasting oven - fairly high up to crisp the crackling.
Contact the owners of the cottage and ask them if Mary Berry's aga book is there. Could be worth asking them if they've got Aga pans, too. They're good as the lids are really tight-fitting, so good for steaming veg.
Hope you have a good weekend, and if you need any other help, just ask.