Villiers D270 oil leak

Hi Folks. For some years now, I have owned a 1965 year old Villiers D270 single cylinder diesel engine fitted to a BKB alternator. It is permanently connected to the house in case of power cuts. An oil leak has developed from the oil seal at the flywheel end. There is a long 5/16" bolt running through the hollow shaft to secure the rotor, presumably on a taper? At the roller bearing end, there is a 1/2" BSW tapping. I presume this is to assist removal of the rotor from the flywheel by inserting a threaded rod to push against the crankshaft? This is a rather strange one to me. Can you give advice, please?
Gus. 

I have a BSA 220 geny set and to remove the rotor ther is a long boult runing through the hollow shaft pulling onto the engine. To remove the rotor i inserted a shaft between the long screw and the engine crank. This has to be larger then the thread.It took a lot of force to prise them appart!! Not saying yours is like that but may just help.

Many thanks, dunitrong. You have confirmed what I suspected. I have cleared the 1/2" BSW thread, and have ordered a bolt to push against a bar up the shaft, but it will not arrive before Wednesday, so will let you know then how I get on. Gus.

My Whitworth bolt arrived yesterday to help with withdrawing the rotor. I have since remembered that a lot of machinery, and the Villiers in the 1960s had UNF and UNC threads. Not to worry, as the Whitworth bolt has the same diameter and TPI, but slightly different pitch shape. With a turned down spindle inserted, the rotor pulled off the taper, but still the shaft and disc remained firm. A home-made puller and some heat shifted this, revealing a short straight shaft well held by a little rust. Modifying the puller, the flywheel eventually gave up, revealing the oilseal. Removed using two self-tapping screws.

Nicely done are you going to replace it with one of the modern double lip seals?

Hi Stuart. No, I have ordered an ordinary one from Simply Bearings.   
Just inserted another pic that got lost in the last post. It shows the back of the disc, with the short straight locating peg that was slightly rusted in, and the cause of all the fuss.

Actually, when the new seal arrived, it was a double lip. Problem now is that the generator will not excite. Checked all connections OK. Oil contamination on brushes, slip rings and comm. Cleaned them with meths. On one run, test light went bright, but since then, just a glimmer. Still evidence of black type deposits. Need advice, please.