In reply to jkmurray:
A Base Layer means exactly that - The base layer of clothing against your skin.
These days it has (wrongly) become synonymous with a long johns style long sleeved merino wool or synthetic top.
Not sure what your list from Macmillan looks like, but for a night-time Aug hill walk, I'd say -
1. Walking boots
2. Any trousers (not Jeans or other slow to dry material) - Light is best
3. Water proof trousers - for rain or if cold
4. T-shirt - cotton is fine, despite what many say
5. Jumper - light and quick to dry if possible
6. Water proof jacket - for rain or if cold
7. Hat & Gloves (yes, even in Aug)
7. Rucksack with spare jumper, food, water, map, compass, first aid kit, survival bag, head torch and spare batteries, mobile phone with full charge.
As long as you've got boots and a good set of waterproofs, everything else can be 'nothing fancy' style stuff in the back of your cupboard. No walking poles required - they're nice to have, but not necessary.
If this is your first hill walk, I'd recommend doing going up another couple of hills of similar size during the daytime before aug.