Can't help with the seatclamp size I'm afraid, only to add that it will be a bigger size than the seatpost
If the post is 30.2 then the seatclamp is likely a 34.9 or something (taking into account the thickness of the wall of the seat tube), but don't quote me on that.
I can reccommend getting a cheapish set of vernier calipers, you can get them for around £10, and they come in mega-handy in situations like this
ps: does it not say the seatpost size on the seatpost? or the seatpost clamp size on the clamp itself?
Cheers for that, it had the most mangled seatpost (which had been drilled out), no clamp and the top of the post is rather mangled. Will borrow some digital calipers and measure tube down where it's tidy.