Right I've got a boardman mountain pro and the rear disk has had it so instead of buying a new 160 disk I was going to use the one of the front wheel and change that to a 180 so my question is will it fit. Its a post mount dunno if that makes any difference.

Cheers for the help it's always appreciated.

Yes, you will need an adaptor like one of these CRC Post Mount Adaptors page and they are pretty universal - just make sure it's the right size and manufacturer.

Pretty simple to swop over also, 5 min job I reckon and an allen key