This is an important theme, correctly handled:
On take off at about 300 ft the whole aircraft started shuddering violently with severe loss of power but still able to hold altitude but not climb , straight ahead landing would of written the aircraft off as it was a small rocky valley so I flew a 300 ft circuit but prepared to put the aircraft down straight ahead at anytime
These symptoms accurately describe a "stuck valve" which is reasonably common (yes, even on low time engines). When things start shaking up front, it's very important to actually determine has the engine actually stopped developing power entirely, or is it just running rough, and shaking like heck?
"stuck valves" take several forms, but as the pilot, all you need to know is that you have either 25% (4 cyl) or 17% (6 cyl) less power, but the engine is still devoping the rest of the power. The shaking is just because the power is no longer balanced. If the valve stuck open, you have some maintenace in the engine's immediate future, but you're still flying. If it stuck closed, something expensive is now bent, but the engine is still developing power - for a while. (a spark plug blowing out, has a similar effect)
Valves seem to stick a few hundred feet up just after the first takeoff of the day. I suppose that's when the heat of the power being developed finally has it's effect on all of those moving parts. Just because the engine is shaking, don't just abandon remaining airborne! Try and fly! (Okay, if you have a perfect runway right ahead of you, you should land on it, otherwise don't just force land for no good reason!) I've known several aircraft which were carelessly and injuriously crashed, when the only problem was a 17% loss in power and some shaking. All certified aircraft, which are being properly flown, will climb at least somewhat, on 83% power.
At the risk of being resoundingly slagged again, I will suggest that when conditions are suitable and safe, pilots practice a full takeoff and climb with 75% power (knowing that you can go to full power at any time during the practice). You will be surprised how well most aircraft will get airborne doing this, and you'll have a sense of how your aircraft will fly if you do have a partial power loss one day.
If the engine is developing some power, make the best use of it, don't let the shaking bother you, the plane can take it. (It's a "pan" not a "mayday"!) I've climbed away on partial power, to return for a safe landing many more times than I have had a complete engine failure, and force landed. I have been very lucky in never having damaged an aircraft in any of my four forced landings, or landing into a place from which a takeoff later was not possible.
Pilot DAR