Renault Megane clunking exhaust and feels like handbrake on when driving off

poss cat breaking down this would account for the noise and poss a lack of power as the cat blocks.!

The holding back from parked as if handbrake was on is probably due to sticking rear brake shoes. The fact that driving back produces a clunk, and then all is ok is a big clue. Perhaps the car was laid up prior to you buying it.
Usual fix would be to remove rear brake drums check brake cylinders are not leaking, remove rear shoes all adjusters and any linkages. Clean and inspect everything. Replace any heavily corroded or seized components. You can buy kits from most motor factors that contain every mechanical part required to overhaul the rear brake shoes. Replace rear wheel cylinders and brake shoes if required.

Whilst it is all apart check that hand brake cable is not sticking.

I am assuming your car has rear drums not pads!

To try and find the knocking at the back, with the exhaust cold try, banging it upwards along its entire length. This is much easier if you have access to a pit of four-post lift. The car has to be level with all four wheels on the ground for the test to be worthwhile. Any Quick-fit will do this for free; you may only need the exhaust system rotated slightly.

The cat is near the front of the exhaust system again with the exhaust cold (very important if you do not want a severe burn) tap the cat firmly with your hand if it rattles it may have broken up inside- it may only be an external heat shield that is rattling.
The first answer (post) suggests the cat could be braking up and causing the exhaust to block, if this were the case you can usually hear a tinkling sound as you drive and bits of the cat make there way down the exhaust. However from your description sounds like your rear brakes need an clean out or overhaul.