Thanks for the reply...
Mine is different in appearance to your pictures but probably similar in design? .. although when I remove the blade disc there is no gap between the drive bearing and the deck so impossible to "feed" a belt upwards to the pulley above the deck.
I think now I have removed the blade disc I may as well release the motor bolts and fit the belt from above - although I don't recollect having had to do this before!
Thanks again