Ian,
I presume this is your first venture into national commercial work, what you are experiencing is VERY common.
The red tape that needs to be cut through in order to get an invoice signed off can be unbelievable, we have a few contracts that work on strict 90 days terms, so you need to build that into your price.
You will also find that they will find any excuse for not paying, however they will not contact you to inform you of a problem, they will wait until you ring to chase up the invoice!
Invoices get "lost" in internal mail, local branches blame head office and vise versa and the whole process has to start again.
If you start encountering problems the best thing I have found is to work with their accounts payable department (you get a better response) if they need another copy of your invoice, fax it and also send a recorded delivery to ensure it has been received. Also speak to your local contact (friend) and get him to chase it up from the inside.
I generally contact them a week or two before payment is due to ensure they have it on their next payment run.
I have never yet had to resort to "final demands" or small claims courts but I'm sure I've got close to it.
I currently have two domestic customers who have outstanding invoices for roughly £200 each, which are both heading towards final demands and threats of court action.
Luckily my wife is a management accountant in one of these same businesses, has done many years as a credit controller and is very tactile on the phone to accounts departments, she chases up payments for me and generally gets payments through pretty quickly.
Merry Christmas