I have recently installed Jessie on my HDMIPi, and the first thing I have tried to do is to install simplecv, but every attempt so far has failed. Now there is most likely a load of rubbish in there that it would be nice to get rid of. Is there a way to go back to the situation just as it was immediately installing Jessie, what I might call a "factory reset"? I can't simply re-install from scratch (can I?) because the Noobs SD card will have been overwritten. I have nothing I want to retain from what I have done so far, and I assume that trying to un-install will not succeed since nothing installed properly..
Raspbian is an image which can be downloaded as a file and written onto an SDCard of adequate size.
Essentially, disk images are files which contains everything that will exist on the new disk. Including the formatting. A disk image can be everything on a hard disk drive, a new CD ROM, a new DVD or even an SDCard.
Consequently, writing a disk image to media such as an SDCard will make anything formerly on that SDCard, at the very least, difficult to access or, at the very most, completely over-written.
So, when using a Raspberry Pi, to "start over" simply write the Raspbian disk image onto your SDCard. You will need to use a special disk image writing application. But disk image writing applications are easy to find. Remember to copy anything you want to keep before doing so.
Noobs is a set of files that are copied onto the 1st (usually) FAT formatted partition (perhaps the only portion) on an SDCard. Guessing, if the Noobs files are the only files on the 1st partition which is (usually) formatted as a FAT partition, it will probably overwrite the entire SDCard with new data.