Spitfire 1300 engine problem opinions please

[kartman24]

The engine in my car is getting very tired, it has compression between 85-95lbs on the 4 cylinders and it ticks over at 900rpm but is quite lumpy, if I remove the oil filler cap or the breather pipe from the rocker the revs jump up by 600rpm. Putting my finger over the breather pipe on the side that goes to the inlet manifold reveals that the pressure is sucking from the inlet manifold. The engine is using oil, 4.5 litres on the return trip from Le Mans Classic, my initial thought was rings but would that cause the sucking from the breather pipe connected to the inlet?............Martin