hi have you taken the breather or oil filler cap off while the engine is running ? if theres oil flipping out or smoke puffing out that means you have a lot crankcase pressure which means compression is getting past the rings .If you took it to a garage you should be able to get a compression test done for about 30 mins labour cost, or you can buy one at machine mart for about £25 and you can test your valves as well,
i dont know if its still available many years ago you were able to buy piston seal have a look around the better accessary shops , if you are constantly topping up oil i"d say rings.
sorry thought it was a petrol car getting old you know, you can still get a compression test done on a diesel but its likely to cost double ,and dont buy a tester they cant cope with diesel compression.