Thanks David. The developers have come back with this point, do you agree with it? Is it Standard ERP which gives these tokens, or is it the Standard ID authentication server?

"I have also proven another issue with Standard ERP which has caused us significant problems and delays in development. Every two or three requests are rejected and we get an empty message returned from Standard ERP. If I put a break point between the request for the access token and the request that sends the data and let it wait 2 or 3 seconds it works every time. It seems to be a race condition on the API where it's not accepting the credentials fast enough. We will need this fixing before we are able to go live as this is stopping the required request from completing. I have unit test that can generate this problem, it seems to stop responding after 3 requests."