Welcome back Snuggly It is lovely to see you again.
Stuck welcome to the frugaleers . £100 pm is doable. We did £240 pm for 6 long years for 4 people.
If you don't already shop in Aldi or Lidl and Iceland - not because there aren't cheap things the other but because day in and day out it becomes harder to resit the temptations.
Meal plan - we did ours around the super 6 vege in Aldi. You will find you end up eating more seasonally - which has the added environmental benefits.
Learn how to make the "specialty" items - wraps, chapatis, naans, chutneys, beer, bacon, dressings etc. Which are all really simple to do but expensive to buy.
While you still access to the joint income I would suggest getting yourself to Indian and Chinese supermarkets & getting a really good store cupboard set up - that way you can make anything from really cheap ingredients. Google your favourite takeaway recipes and pick up those ones. If it helps I can list what is in mine (although I am a tad obsessed)
Keep a couple of store cupboard meals in - just in case you run out of money before the end of the month. I still do this (made easier b/c of the spice cupboard)
If you have room set up a vege bed / put part of the garden over to grow expensive things like herbs, salad leaves, radishes, beans, tomatoes and soft fruit. None are difficult or expensive. I just put all of mine in and around the garden.
If the £100 is to cover non food groceries we found it helpful to buy all of those at the beginning of the month. Obviously avoid branded items - if DD is picky you can decant them into the containers you already have.
wreck DS is busy teaching himself keyboard, sax and guitar from You Tube. Like all things - practice, practice and a bit more practice.