As above, using these on the standard alloys, to give them a slightly better stance. . Was thinking 8mm front, 15mm back, will I need longer bolts for either?

Thanks

Yes, in any case you will lose thread contact and leave a weaker connection. Ideally your bolts should be as much longer as your spacers are thick. Buy a set of decent spacers and they should come with bolts.

Where can you buy longer bolts? How long are standard?

Can usually get longer bolts from the places which sell the spacer kits. It kind of makes sense for them to be able to supply absolutely everything you need. Standard bolt are M12 width and I think between 45 and 50mm of thread. I used oem bolts with my 3mm spacers, but there was still plenty of meat to screw into in that case. Any longer and i would have been looking for new bolts too.