Thanks for the replies
I have tried it with the internal link as the webtop or as a portal. I believe the BigIP acts as a proxy either way.
Access works fine when using a network access tunnel.
As a portal, the internal page displays correctly but there is a link on the page that will not work. The page is partially displayed when the link is changed to http. So I guessing that the mixture of http and https are causing a problem- when the link is https some elements of the page are not secure so it does not like it, but I can't see anything in the logs.