its a late 40's ex-council house in scotland - any ideas how to get a copy of the original plans ?

In with your deeds...?

should have said, i've got a copy of the land regestery entry an all it shows is a 1:200 map with a few dimensions of the building outline....

I seriously doubt you'll be able to dig up anything better - a while ago I had a bit of a 'foreigner' doing As Built drawings for a mate of mine who owns an Electrical Contractors and who got a contract to rewire 2000 odd Scottish council houses. I couldn't get any drawings out of the councils involved and that was as a contractor working on their properties - to be honest I don't think they were being difficult I just don't think they had them any more (if they ever did!).

Phone the Council Planning Dept and ask if they have a copy.
If they do, they ain't cheap.