Accessing the internet with knitr - google-maps

Question

When I run some code to draw a map using ggmap, in Rstudio it runs fine. When I run it using knitr it fails with the following error message:-
Error in download.file(url, destfile = destfile, quiet = !messaging, mode = "wb") :
cannot open URL 'http://maps.googleapis.com/maps/api/staticmap?center=-40.851253,172.799669&zoom=19&size=%20640x640&scale=%202&maptype=hybrid&sensor=false'
Calls: ... eval -> eval -> get_map -> get_googlemap -> download.file
In addition: Warning message:
In download.file(url, destfile = destfile, quiet = !messaging, mode = "wb") :
unable to connect to 'maps.googleapis.com' on port 80.
Execution halted
I am sure this is due to the way our network is set up, probably around permissions, but is anyone able to give me any clues as to how knitr would try to access the internet to download a map so I may be able to find a way though our firewall.
Code added but it works fine except through our network.
---
title: "Drawing a map"
author: "Alasdair Noble"
output: word_document
---
To draw a map
```{r echo=TRUE, warning=FALSE , results='markup', comment="",message=FALSE }
library(ggplot2)
library(grid)
library(GGally)
library(plyr)
library(RColorBrewer)
library(ggmap)
library(ggthemes)
```
```{r echo=TRUE, warning=FALSE , results='markup', comment="", message=FALSE }
Btrcup <- get_map(location=c(lon=171.799669, lat=-42.851253),zoom=19, maptype="hybrid")
Btrcupmap <- ggmap(Btrcup)
Btrcupmap
```