More filing and stuff, marked and drilled the holes in the metal plates, they seem to sit in the slots OK-ish. I'll have to come up with some patent bull$hit like "Ferr-O-Tone solid steel Sustain Plus Blocks"
A trial fit looked OK, strung it up to see what sort of action it might give, and had a nasty moment when I thought the truss rod nut had stripped it's hex socket, a search thru my tin of Allen keys was unsuccessful, however I tried the one that came with the Squier P bass and it fitted, so was able to do it up a bit ( I'd let it right off when disassembling the thing originally) will no doubt be tweaking further.
I'll leave it to sit a while now with tension on the strings, I did them up in stages, they're a semitone flat from standard now, which is what I generally have basses tuned like (unless I've got to play with other people who won't tune down)
I'll attempt to get it so its action is kind of playable before messing with the next bit, putting PUs in etc