After yesterdays disappointment of finding that the Green Cortina was still leaking oil out through the dipstick tube, I went out for breakfast with Gavin and we discussed the issue thoroughly.
When I had this problem originally, I posted a question on a Facebook page as to what could be causing it. Some of the answers were helpful, some were very negative and some were just downright ridiculous, so I was nowhere near an answer. A few people said it was crankcase compression caused by knackered piston rings, but my engine doesn't smoke, doesn't burn oil, doesn't have an oil haze floating over it and doesn't even smell oily, so I really can't believe that my engine needs a rebuild.
Once back home, we started investigating a bit more. Gavin was reading through the workshop manual and found that the oil capacity of the engine should be 6 pints, or 3.5 litres. The only way to check that that's what was in the engine was draining it out. So, I made sure that the oil was reading maximum on the dipstick, and the drained all of the oil out. (Including the what was in the oil filter)
To my surprise, all I got out of the engine was 4.5 pints. So that meant that somehow, the dipstick wasn't reading correctly. After even more investigations, we found that the measurement of the dipstick tube from the timing cover to the end of the tube where the dipstick enters should be 5 inches. Mine was only measuring 3 inches. So, from what we can make out, a previous owner had fitted a dipstick tube from a 1300cc engine instead of a 1600cc engine. This will also explain why my dipstick reads maximum oil level when its really 1.5 pints short.
Luckily, Burton Power Products sell the tubes brand new, so I've now got one on order. Hopefully, you never know, this might, just might cure my problem.