Sara, how long is a piece of string? If you only intend on car camping then take the bloomin lot! But if you are just getting back into the swing....take the bloomin lot.
In other words make sure you are as comfortable as you can possibly be, I would definitely recommend a sleeping mat and/or a Thermorest.(I take both).
Your old sleeping bag should be fine and if it has been stored fairly well will certainly do you for the summer.
As you say many sites will even have a cafe on them but they will be extremely expensive and will not provide you with a cuppa at all hours so I would definitely take a cooker,cup, 'T'bags/coffee bags, ( I don't do instant),cuppa soups and drinking chocolate. Comfort again.
You ask about sites there are sites where you will get a porta loo and a cold tap right up to sites with heated loos, showers,a bath, full laundry facilities a club house/restaurant/pub.
You don't say whether or not you are going to walk or just sit around the site relaxing?
That should get the ball rolling for you, others will be along shortly.
you ask for a list ok....
Rucksack
tent
sleeping mat
thermorest
sleeping bag
silk liner
pillow
cooker & fuel
cup/mug
pans
fork & spoon
water bottle
washing kit
Towel
Flannel
clean clothes
warm top
this is probably the minimum, some would say not complete because there is no bottle of wine, then of course you will require a glass.
The most important thing is if you are not backpacking you can take it. Things get tricky when you have to decide on what your minimum is if it is all going into a rucksack and you will be carrying it up hill and down dale.