T9 leaks are often from the speedo drive. You might be able to seal it, but they often leak. If it dribbles a tablespoon or so overnight after a drive, then stops, it is probably draining from the speedo. The space where the oil collects is pretty small, and it would typically take a year or so before I'd have to add any more oil.
I also have the 10mm allen plug on the side opposite the speedo drive. I picked up a set of low profile allen sockets similar to those linked below. They slip into any spanner, and make access to that fill plug pretty easy.
I marked a dipstick so that I could tell "full" by accessing the top of the gearbox only. It's easier for me to fill from above than trying to get a hose into the fill plug each time. Other members have cut and welded an opening into the top of the gearbox so that they don't have to remove the bolts on top. But I find removing them from the engine bay is pretty easy with a small socket.
[edited for correct website]
http://www.autobodytoolmart.com/e8211z-red-low-profile-hex-bit-set-metric-hm1000-p-29577.aspx?kpid=37029&gclid=CKKq2MmIxMECFQ9cfgodd5AA4g