Adjusting the trim tab in the opposite direction that the boat heads off in usually fixes the problem.  Check and see if the engine is mounted centrally on the stern and not off to one side.

 trim tab as in the sacrificial fin like anode above the prop?? the old boy replaced his a while back and had it over 1 notch or sonething and the boat alwats went hard right

Silly thought but is your overall weight somewhat evenly distributed amidships?.. generally the faster you go the more pronounced things like that might feel, same in principle to a car I suppose. I'm not a boat owner but if your motor is centered and trim tabs are level then logically there's not much else it could be.. maybe a possible issue with the hull?.. If water isn't planing evenly around and under the hull it could create a kind of low pressure system like a airplane wing and maybe pull you?. Sorry if this is all stupid just trying to through out some thoughts.

Had a similar issue with a previous boat. Is it also hard to steer left?

I checked steering cable, helm, steering arm motor etc etc etc..... All good.

Play with the trim up/down while under way. Our previous boat had the pin in the trim holes so you couldn'd trim it too far down so "I'd drop the motor and go".

If trimmed too low it can give similar symptoms you describe.... once I trimmed up after "hitting bottom" all was OK

Cheers guys. I've already got the trim tab hard right and its made no change. I'm gona try drop the motor an inch as it improves when I tilt it up but I start getting pretty high with regards to cavitation plate and bottom of boat. Also might try an oversized or bigger trim tab. The boat has deffinitely more weight on the right side but with two grown men standing on the left side there is no change and it still pulls hard right. Motor is centered and vertical.  So I can't think of anything else but maybe a combination of things making it happen. Height of motor, distribution of weight in boat, engine torque, prop size and pitch maybe? Let you know how it goes. Cheers 

 Pulling right so put your trim tab left. 

Yeah sorry meant pointing left. Trailing edge to the right. I tried it both directions with no noticible change. Problem  goes a little deeper than the tab.

ASfter much searching on the net and trying everything I changed the prop. The boat came with a spare that was damaged. The pitch on the two props were slightly different. I decided to get it fixed and in preparation found it was a genuine prop. With the genuine prop repaired I fitted it and immediately the problem was solved. I only had cable steering but it was impossible for my 10yr old child to turn it to port.

So, the cause of the problem was the torque of the engine acting through too high pitch caused the boat to 'tilt'. Check your prop pitch then inquire what pitch it should be and get the right prop. Chances are the previoius owner replaced the prop with one of incorrect pitch. You can then keep the second prop as a spare to get you home.

 yep trim the motor and make sure stuff in the boat is evenly distributed

embrace it and rename your boat Zoolander... he had issues turning left as well...

Cheers prego sounds like your on the money. Will try a different prop. Zoolander great film.