Printer prints test page and direct prints fine but anything sent from the PC only prints (at most) a thin line (about 2cm/1in) on the left side of the page.

Well, if it ran okay with Windows 8.1 until the last month and if it prints a test page ok, the problem is unlikely to be drum wear or wear of internal parts. You sometimes encounter weird streaking effects if the windows are not clean. Normally, if only one side of a page is being printed, parts are worn or the drum need replacing. But your test page prints out. The only thing I can see that would have changed is --Ta da! -- some automatic update by Windows 8.1. Re-installing drivers doesn't really undo any changes in Windows 8.1 itself or whatever you have installed in the last month or so. (Windows 8.1 experts can stop laughing--I am simply making an educated guess.)

If this were a home setup rather than a work computer, I might see what would happen if I returned the computer to a state prior to "the last month or so." If the printer then worked, you would know it was not a hardware problem but rather what had been installed on the computer after that. Is the printer actually using the Brother driver or is it deferring to a Microsoft driver? Just something for you to check in your device manager. If returning the computer to a former state fixes the problem, you may have to test subsequent installations to determine which one troubles the printer, a tedious process unless a candidate springs to mind.