Base URL for logging into the App

[Steveinjava]

I utilise https for a clients site and I am having trouble implementing administrator login as a client.
The admin module config page has a field called "Base URL for logging into the App"
and I have that set to https://www.clients domain.co.uk:8443/konakart/AdminLoginSubmit.do
It results in an invalid certificate.
I have tried missing out the port number and even using http without success.
Fingers crossed somebody has come across this?