Starter motor problems....

It seems i may have the wrong starter motor, the one i have is a lucas LRS543 on the advice of someone in a internet forum. I bolted it up to the engine earlier and it looks like it is perminantly engaged with the flywheel, which will mean it will be spinning all the time. I have just read on another website that u should use a lucas LRS707 I will do some more investigating tomorrow.....