Basically i have a very stable bashed patch of a large number of mods (FCOM plus many others) and i don't want to risk messing things up if i rebuilt it again, i only want to make minor edits to race records and stats.. can i do it in tes4edit or are bashed patches not supposed to be manually edited?

Exactly. There is no risk to rebuilding it again the same way, it will come out exactly the same except that any edits you made yourself to it will be lost.
Instead, make your changes in your own file in Tes4Edit, you can even load it after the bashed patch if you deem it necessary.

Is this all something that you could do in your own .esp that can then be merged? That way a rebuild in future would just pick up your changes again.