A run of code within an rmd notebook produced rather a lot of console output. Although the script continued running, the console stuck and I had to eventually kill the process when I knew the script had completed.
On restarting Rstudio became non-responsive again. After trying several times, rebooting, sacrificing farmyard animals and surplus offspring, I found a directory within ....Rproj.user\shared\notebooks that seemed excessivly large and seemed to contain the console output. I deleted the directory, restarted, and Rstudio has stopped being petulant.