so I finally got round to melting the plastic bottom bracket out with a blow torch, a quick stamp over a brick and job done !
On closer inspection the thread on the bb looks damaged, I think someone has had this problem before.
I used the blow torch on the seat lugs too, along with a chisel to spread it back into shape.
Measuring the seat post is tricky without the proper tools but I tried a 27.2mm post and a 27mm post and both are too large, measuring the corrected seatpost-hole it looks like a 26.something I'll need to try a 26mm post when I can pick one up, maybe I can use that with a shim ?

I suppose this could be a clue as to the maker ? looking at sheldon browns seat post size database, there seem to be very few 26's quite a few french and italian 26.2mm and a whole bunch of 26.6's including Dawes. With having to reset the crushed lugs it could be any of these.