Good Luck Simon.
As a small business with smaller overheads you can make money where the bigger boys cannot.
I was only talking to someone the other day as to how they had walked away from one of the companies you list.

I also worked for a company who walked away from the same company.
But in fairness both companies are not small.

Please check out the payment terms, better still if you already work for these companies then you are better acquainted with them!
I lost my last business because although I had lots of work. NO One was paying me on time!
So don't extend yourself to far past that which you can afford to loose.
But good luck all the same.
I hope it goes well for you!
Your email implies that you are Cornish which is a big Bonus when working in Cornwall!