Mike is however correct when he says that an MOT cert has no bearing on Road Worthiness in the event of a claim. If after an inspection, the machine was declared not roadworthy, it would still be covered but the amount paid out against the claim would probably be lower than if it was roadworthy.
So, if your broker tells you its not insured due to no MOT, and you had a mishap. The actual insurance company would still cover you as stated above.
I've said it on here before. Insurance Companies are NOT in the business of trying to get out of paying out in the event of a claim. They honour their obligation in respect of the individual circumstances regarding the claim.