Advice for a complete rooky

Hi. Just bought a 2.25L SWB Diesel L Reg Series 3, and can't find where to top up the oil (!).

The Haines manual shows a picture of an oil filler tube. My engine seems almost identical to the one pictured, except there is no oil filler tube. - Just a blanking plate.

Was the filler tube deleted on later models? If so, how do I top up the oil? The only thing I can see is a cap where a breather tube connects to the rocker cover. Do I remove that cap? If so, how? (I think mine has the 5 bearing crank, if that makes a difference , but not even sure of that).

All advice gratefully received.

Black cap with breather coming off it, have a look at that

Ps before you get slated you need to introduce yourself on the newbie section