toolstation do BZP M10 coach bolts in various lengths; about £1.50 for a pack of ten. (eg #25563 M10 x 65mm, x 10off £1.35 ) You will need to buy your own nuts and washers.
NB these bolts are not full threaded, so can only be shortened a certain amount. IIRC they have about 30mm of thread in most sizes.
cheers