Unfortunately, I am not familiar with the brands of kitten milk replacers in England. Here in California, the most reliable one is KMR powder (same makers of Puppy Esbilac) which can be mixed with spring water. Here is where I strongly recommend
against Hartz Kitten Milk Powder -- that one went odd a few years ago and was speckled with blue dots, altho it wasn't toxic.
I can only suggest that you devote a lot of time to the kittens and mama cat, placing the kittens on the milkbar every 3 hours. I would even sleep on a mat with the mama cats and their babies to make sure they fed. If any of them will not milk from her, they may have tummy trouble.
I had one kitten that didn't milk as often as the others and was always behind one meal a day. I have him to this day and he sleeps on my bed in the same curled-shrimp position he did as a newborn when he slept while the others ate. He eats terrificly now altho remains the runt of the males.
All kittens should be round in the tummies at all times and have bright mews. Of course, some kittens won't speak much. If there is any question about their health, take the kitten to a vet that is good with baby kittens.
Also, try to get the mama cat to settle into a cozy, secure environment, such as under a desk, in a tight corner, or in a bathroom or closet that has just enough room for the kittens to live for 5 weeks. It's a good idea to put a little light over the kittens and mama cat at all time - I use christmas lights or a low-wattage desk lamp.
If the mama cat is too anxious, you might need to look into calming supplements . . . or calming herbs, such as a small stuffed toy you could add Passionflower or Lavender buds to.
All kittens burst out of their nests at 5 weeks. Good Luck with the kits!

I L

ve Kittens!