"Your insurance does not cover the following.
Any liability, loss or damage arising while any motorcycle covered by this insurance is being:
“kept or used in an unsafe or unroadworthy condition or without a current MOT certificate (if one is needed) “
My emphasis. I take this to mean that if you don't go for a "non-MoT" you will still be covered but that the usual onus is on you to keep it safe and roadworthy whatever the age of the bike.
These terms of course can be nitpicked to death especially in the case of very old machines. Wasn't someone killed in the car London - Brighton run last year due to getting out of control on a downslope?