Cost balance
The C150 is almost dead as a training aircraft because the maintenance costs are increasing with age, the C152 had most of the major maintenance issues that the C150 sorted out and so continues to bring lower maintenance costs than the C150.
The cost of fuel is the big issue with the C152, it should be burning about 25 LTS/ hour so that is roughly £50/ flying hour, about £30 should cover insurance and maintenance so that leaves enough if the aircraft is charged out at £120-130 to cover the airfield charges and make a living wage.
The big problem for the new aircraft is that the insurance is going to be at least double ( and that is assuming a sorted C152 with a hull value of £32K) it could be trebble if the C152 is a £17K dog ! Add to this the cost of a new £80K aircraft and you can see that the numbers are still stacking up in favour of the C152 inspite of the cost of AVGAS.
It is difficult to predict the cost of maintenance that the new aircraft will need in the next few years, I suspect that this will go up alarmingly with some types, the tecnam seems to be one of the better built of the newer aircraft so I don't see the costs climbing too far with that type.
So in conclusion I would predict that for the forseeable future the C152 will continue to be the PPL trainer of choice.