Hello Kid, thank you for your response.
In reply I have actually ruled out office 365 as an issue now, as this behaviour is seen for all SSL based traffic, not just o365. To further the mystery, this only affects my Windows 7 clients. My 30 x server 2008r2 servers are not effected by this issue at all (they are on the same IP subnet and use the same IP settings) but my 25x XenApp 6.5 servers are affected by this (which are also on the same IP subnet)
Since I have ruled out office 365 and understand that this is now probably the wrong forum to ask for help on this can be set as answered/closed.